Plagiarism is using another person’s work — their words, ideas, research, code, images or data — without giving proper credit, so that it appears to be your own. Stanford University’s Honor Code guidance defines it as “the use, without giving reasonable and appropriate credit to or acknowledging the author or source, of another person’s original work, whether such work is made up of code, formulas, ideas, language, research, strategies, writing or other form(s),” and adds that verbatim text from another source must always be put in quotation marks. Two points in that definition decide most real cases. Credit is the whole issue: the same passage, quoted and cited, is scholarship; uncited, it is plagiarism. And intent is not part of the definition — careless plagiarism is still plagiarism, which is why this article spends as much time on the accidental kinds as on the deliberate ones.

Plagiarism, defined properly

University definitions agree with each other closely, because they are all policing the same act. The Harvard Guide to Using Sources states that plagiarism is submitting work written by someone else, intentionally or unintentionally, and that including material from any source without citing it is plagiarism. The Harvard College plagiarism policy fills in what citing requires: quotations must sit inside quotation marks and be cited fully, paraphrased material must be acknowledged completely, and ideas or facts taken from your reading — or even from your own earlier writing — must be attributed. “Sources” there explicitly includes information and opinions gained directly from other people, not only published text.

Behind the definitions sits a set of shared values. The International Center for Academic Integrity defines academic integrity as a commitment to six values — honesty, trust, fairness, respect, responsibility and courage — and plagiarism is treated as a breach of integrity, not a formatting error: the offence is presenting a false picture of whose work a reader is looking at.

What plagiarism is not is also worth fixing early, because three neighbouring ideas are constantly confused with it:

  • Copyright infringement is a legal offence; plagiarism is an ethical and academic one. More on the difference below — they overlap, but neither contains the other.
  • Common knowledge needs no citation. Facts your audience already knows, or would find in any general reference, are yours to state plainly.
  • Collaboration is plagiarism only when it crosses the rules set for that piece of work. Harvard’s policy makes compliance with each course’s collaboration rules part of the same obligation — help that is permitted is not misconduct, and help that is not permitted is, even between friends.

The main types of plagiarism

Labels vary slightly between institutions, but the behaviours below are the ones university policies and guides consistently name.

Direct (verbatim) plagiarism

Copying text — or code, or a passage of music — exactly, without quotation marks and without a citation. This is the clearest type and the easiest to detect. Changing a few words inside a copied passage does not rescue it: Harvard’s guide treats copying language word for word without credit as plagiarism whether you quote it, and the mosaic type below covers the lightly edited versions.

Mosaic plagiarism (patchwork)

Weaving phrases and sentences from one or more sources into your own paragraphs, swapping in synonyms here and there, without quotation marks or proper paraphrase. The Harvard Guide to Using Sources describes mosaic plagiarism as copying bits and pieces from a source or several sources, changing a few words, without either adequately paraphrasing or quoting directly. It is the most common student type, because it often begins as note-taking: source sentences land in a draft, get lightly reworded under time pressure, and are submitted still wearing the original’s structure and half its wording.

Paraphrasing plagiarism

Putting a source’s ideas into genuinely your own words — but without citing the source. Correct paraphrase has two halves: new wording and a citation. Students routinely remember the first and forget the second. An uncited paraphrase is plagiarism even if not a single word matches the original, because what was taken is the idea, and ideas are exactly what citations exist to credit.

Self-plagiarism

Submitting your own earlier work — a whole essay, or substantial parts of one — as if it were new, without disclosure and permission. Harvard’s policy expects sources to be indicated when ideas or facts are derived from “a student’s own writings,” and its handbook tradition treats work submitted for one course as done solely for that course unless reuse is authorised. The logic is the same as for anyone else’s work: the reader — here, an instructor awarding credit — is entitled to know the work is not what it appears to be.

Accidental plagiarism

The umbrella for the unintentional cases: a citation dropped in editing, notes merged into a draft without their source tags, a paraphrase that drifts too close to the original, a missing set of quotation marks around a short quoted phrase. Note what the definitions above do with intent — Stanford’s and Harvard’s both judge the act, not the motive. Accidental plagiarism is usually treated more leniently than deliberate deception, and it is still recorded as plagiarism. Almost everything in the “how to avoid it” section below is aimed at this type, because this is the type careful students actually commit.

Source-based and fabricated-source plagiarism

Citing a source you did not use, citing one that does not support the claim attached to it, or — a newer and fast-growing version — submitting references that do not exist at all. Invented citations are a known failure mode of AI writing tools, which can generate convincing, entirely fictional bibliographies; the related failure is covered in what is AI hallucination. Pasted into an essay unchecked, a fabricated reference is the student’s plagiarism problem, not the tool’s.

Examples of plagiarism

Short before-and-after cases make the types concrete. In each pair, the problem is attribution, and the fix is mechanical.

Copied sentence, no marks, no citation. Original: “The credit hour reasonably approximates not less than one hour of classroom instruction and two hours of out-of-class work each week.” Student version: the same sentence, unchanged, in the middle of the student’s own paragraph. Plagiarism — verbatim. Fixed by quotation marks plus a citation, or by a true paraphrase plus a citation.

Synonym-swapped sentence. Student version: “A credit hour roughly equals at least one hour of class teaching and two hours of outside work every week.” Plagiarism — mosaic. The sentence’s skeleton, order and phrasing still belong to the source; only the surface words moved. A real paraphrase restructures the thought (“Federal rules define a credit hour by workload: for each weekly hour of teaching, students are expected to do at least twice that in independent work”) and still cites the source.

Good paraphrase, no citation. Student version: the restructured sentence above, with no reference anywhere. Plagiarism — paraphrasing type. The words are the student’s; the idea is not, and the idea is what needed the credit.

Last term’s essay, lightly retitled. Submitted to a second course without telling either instructor. Plagiarism — self-plagiarism, under the standard rule that submitted work is represented as produced for that course.

A classmate’s structure. Rebuilding your essay on another student’s outline, argument order and examples, in entirely your own sentences, without acknowledgement. Plagiarism. Stanford’s definition expressly reaches beyond language to “ideas… strategies” and other forms of work — an argument’s architecture is work.

What is not plagiarism: common knowledge

You do not cite what your readers already know. MIT’s Academic Integrity Handbook guidance gives the working test: would this idea or fact be familiar to someone like you — a classmate, say — who has not researched the subject? If yes, it is common knowledge and needs no citation; if no, cite it, even if it appears in several sources — appearing everywhere does not by itself make something common knowledge. MIT’s examples of what always needs citing are the ones students most often assume are free: statistics and data, references to studies, and specific facts an average reader would not know without doing the research. Knowledge can also be common inside a field and unknown outside it, so the audience you are writing for is part of the judgement. When in doubt, MIT’s advice is the safe rule for everyone: cite it. An unnecessary citation costs a line; a missing one can cost a course.

The BYU Copyright Office’s comparison draws the line cleanly. Plagiarism is using another’s work without attribution, passing it off as your own; it applies to all source material, including public-domain works, and it is policed by honor codes and professional standards. Copyright infringement is using a protected work without permission (or an applicable exemption); it applies only to works currently protected by copyright, and it is policed by federal law and courts.

So the two can come apart in both directions. Copy a public-domain author — Shakespeare, Dickens — and present the passage as your own writing: plagiarism, and no copyright offence, because the copyright expired centuries ago. Republish someone’s recent photograph with their name proudly attached, but without their permission: possible copyright infringement, and no plagiarism, because you never claimed it was yours. Copy a modern source without credit and you can commit both at once. This is also why a famous court case about a copied song belongs in a copyright lesson, not a plagiarism one — guides that use infringement cases as their headline plagiarism examples blur the very distinction students are trying to learn.

Consequences of plagiarism

Consequences are set by institutions, not by a single law, so they scale with the offence and the setting. In a course, typical outcomes run from a zero on the assignment, through failure of the course, to a formal disciplinary record. At the institutional level, Harvard’s published policy states that students who submit work not their own, or without clear attribution, face disciplinary action up to and including being required to withdraw from the College. Beyond school, the same act ends journal articles (retraction) and jobs in fields — journalism, research, publishing — where a reputation for honest attribution is the qualification. Repeat and deliberate offences are treated far more severely than first accidental ones everywhere.

A modern addition: submitting work generated by AI tools as your own writing is treated by many institutions under the same dishonesty rules when it is unauthorised or undisclosed — the policy question is what your school and instructor permit, so check the syllabus rather than assuming. What generative AI produces is not your work in the sense the definitions above protect, and presenting it as such fits the classic pattern: credit claimed for work done by another source.

How to avoid plagiarism

The practical system is short, and every step maps to a failure mode above:

  1. Keep sources attached to your notes. Record the source beside every quotation, figure and idea as you take notes — not afterwards. Most accidental plagiarism is born in a notes file where a copied sentence lost its label.
  2. Quote exactly, or paraphrase completely. Quotation: copy word for word, add quotation marks, cite. Paraphrase: close the source, write the idea in your own structure, then cite anyway.
  3. Cite ideas, not just words. If the thought, data, structure or example came from somewhere, the citation goes in — paraphrasing changes the wording obligation, never the credit obligation.
  4. Disclose reuse of your own work. Ask before submitting anything that overlaps earlier submitted work, and cite your earlier work where the rules require it.
  5. Verify every reference you submit. Check that each citation exists and actually says what you claim it says — doubly important if any tool helped you draft, because fabricated and mismatched references are now a standard failure mode.
  6. When in doubt, cite. Over-citing common knowledge is a minor style fault. Under-citing a source is the offence this whole article describes.

Frequently asked questions

What is plagiarism in simple words? Using someone else’s words, ideas or work without saying where they came from, so it looks like your own. Quoting with quotation marks and a citation is fine; the same words without them are plagiarism.

Is paraphrasing plagiarism? Paraphrasing itself is a normal, expected writing skill — with a citation. Paraphrasing without crediting the source is plagiarism, because the idea is still someone else’s. Paraphrasing too closely, keeping the original’s sentence structure with synonyms swapped in, is plagiarism even with a citation, because the writing is still the source’s.

Can you plagiarize yourself? Yes — self-plagiarism. Resubmitting your own earlier work, in whole or in substantial part, as if it were new, without disclosure and permission, breaches the standard university rule that submitted work was produced for that course.

Is accidental plagiarism still plagiarism? Yes. The major university definitions judge the act, not the intention; a missing citation or an over-close paraphrase counts even when it was careless rather than dishonest. Institutions usually punish deliberate deception more harshly, but “I didn’t mean to” is not a defence that removes the offence.

Is plagiarism illegal? Plagiarism itself is not a crime — it is an academic and professional ethics offence, enforced by schools, employers and publishers. The same copying can also be copyright infringement, which is a legal matter, when it uses protected work without permission. The two are separate tests; passing one does not pass the other.

Do I need to cite common knowledge? No. Facts your audience can be expected to know — the MIT test is whether a peer who has not researched the topic would already know it — need no citation. Statistics, study findings and specific researched facts are not common knowledge, however many places repeat them. If you are unsure, cite.

This article is general information based on the university policies and academic-integrity sources linked above and checked in November 2026. Every school sets its own academic-integrity rules and penalties — your institution’s honor code and your course syllabus are the final authority for your work.