Substantial Similarity
Definition
The legal standard in copyright infringement: whether the protected expression in one work is reproduced in another to a degree that a reasonable person would recognise the copying. Ideas alone cannot be protected; only the particular expression of ideas.
- Field
- Copyright law
- Applies to
- Infringement determination
- Test
- Whether protected expression is recognisably reproduced
- Excludes
- Copying of ideas alone
Common questions
How does substantial similarity differ from plagiarism as investigated by text-reuse detection?+
Plagiarism detection identifies textual overlap and unattributed copying regardless of legal protection, including copying of facts or ideas that copyright does not cover. Substantial similarity is a narrower legal test asking specifically whether the protected expression, not the underlying idea, has been reproduced closely enough to infringe.
Why does the idea-expression distinction matter when applying this standard?+
Two authors can independently write about the same idea, such as a love story set during a war, without infringing each other, because copyright never protects the idea itself. The test only asks whether the particular way that idea was expressed, its specific wording, structure, or detail, has been substantially copied.
Related terms
- Mosaic Plagiarism
- Borrowing phrases and expressions from a source and embedding them across new sentences, so no single passage is a direct copy but...
- Paraphrase Plagiarism
- Reproducing the ideas and structure of a source while replacing most of the wording, making character-level detection ineffective. Requires semantic rather than...
- Sentence Embedding
- A neural representation of a sentence as a dense numeric vector, trained so that semantically similar sentences land near each other in...
- Translation Plagiarism
- Copying from a source in another language and rendering it in the target language. Defeats monolingual detection entirely and requires cross-lingual semantic...
- Verbatim Plagiarism
- Copying text word-for-word from a source without attribution. The simplest form to detect computationally because character-level matching directly finds the copied string.