Derivative Works under US law and the Berne Convention
US Copyright law defines "derivative work" in 17 USC 101 as:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.
Article 2 of the Berne Copyright Convention reads, in relevant part:
(3) Translations, adaptations, arrangements of music and other alterations of a literary or artistic work shall be protected as original works without prejudice to the copyright in the original work.
Under 17 USC 106 one of the things that a copyright owner has the exclusive right to do or authorize is:
(2) to prepare derivative works based upon the copyrighted work;
Note that under the US definition, a derivative work is one based on an earlier work, not one that merely refers to a derivative work. Merely mentioning or citing a previous work is not enough to make the later work derivative. It must, in some substantial way, use and be developed out of the previous work.
The classic examples of a derivative work are: a translation into another language; an adaptation into another medium or form (such as making a novel into a play or film); and making a sequel. But derivative works can be derivative in various other ways.
Examples from the Question
Let us look at the examples mentioned in the question:
- an academic paper that cites or refers to other papers
This is a mere reference and is not at all derivative. However a paper that contained little or no original content, but consisted almost entirely of such citations might be considered derivative.
- a discussion thread on Internet (or each comment or reply on it) about a work
Discussion or criticism of a work, or a reply to it, is not normally considered derivative. Such comments are not derivative of the work under discussion, nor of the previous comments. Quotations of either are likely to be permitted fair use, and so not infringing, although that will depend on the exact details of each case.
This is likely to be a derivative work, depending on how much of the source work is used, and whether distinctive details are reproduced. Note that ideas cannot be protected by copyright. (See 17 USC 102(b) for this.) So the idea of a person who turns into an animal of a particular type, or has some special power or ability or characteristic is not protected by copyright. But the details expressed in the source work, such as the mannerisms or appearance of a character, or the specific nature of a setting, can be protected, and to draw on them may be infringement, unless fair use applies.
"Metadata" is a very general idea. Typically the kind of information included in metadata is not protected by copyright, but in some cases it may be.
A review, criticism of, or comment on an existing work is not normally considered a derivative work. Quotes used in a review or criticism are often protected by fair use under US law, but if the quotation is excessive, and particularly if it allows the review to serve as a replacement for the original and to harm the market for the original, it may be infringement.
- a work inspired by other works
Again, this depends. If only the general idea is imitated, then the later work is not derivative If detailed, distinctive, and specific characters, settings or plot lines are imitated, then the later work may be considered derivative, and infringing if permission is not obtained.
The Nichols Case
In the case Nichols v. Universal Pictures Corporation, 45 F. 2d 119 - Circuit Court of Appeals, 2nd Circuit (1930) Judge Learned Hand, a famous judge, wrote a very influential and often cited opinion describing when two similar plays are, and when they are not, infringements of one another. It is worth reading to better understand this issue, even though it is over 90 years old.
What makes a Work Derivative
So in general a work is derivative when it uses significant, distinctive, and detailed elements of a previous work, or is a revised or altered version of the previous work.
Copyright in a Derivative Work
If a derivative work is made with proper permission, or the source work is out of copyright, the author of the derivative work has copyright in all the original parts of the derivative work. However if the derivative work is infringing, and was created without permission, the author has no valid copyright in it at all.
Fair Use and True Parody
If a work is held to be a fair use of the original, it is not infringing even if it is a derivative work, this is an exception to copyright in US law. It may still be important to note that the work is derivative, because the author has copyright only in the original aspects.
If a work is a parody, it is likely to be held to be a fair use. Note that in copyright law a true parody is a work that comments on the source work by imitating or mocking it. A work that imitates a source work merely to be funny, or to comment on general social conditions, or to make some other point, but not to comment on the original, is not a parody in US copyright law. Courts often refer to such a work as a "satire", even though this is not the standard literary definition of "satire". A Satire will usually not be a fair use of the source, or if it is, it will be for other reasons.
Addition (in response to an addition to the question)
- original work : poem "Eloisa to Abelard" by Alexander Pope(1688-1744)
- (potential) derivative work : film "Eternal Sunshine of the Spotless Mind" directed by Michel Gondry
The film's title is a quotation from the poem, and the quoted part is cited within the film. Does it make the film a "derivative work" of the poem?
No. First of all, any copyright on the poem has long expired, it is in the public domain.
Secondly, merely referring to a previous work in the title and citing it in the body does not make the later work derivative, it is merely a literary reference. A significant part of the earlier work would need to be used to make the film derivative. Unless much more was used than is mentioned above, this would not qualify even if the poem were still in copyright.
Meaning of "Derivative"
Like many other words of the English language, the word "derivative" has several different senses. The sense leading to the definitions:
- imitative of the work of another person
- inspired or motivated by something
is mostly used in the context of criticism and review of works of art or literature. In copyright law, "derivative" is a technical term, defined by the definition section of the law, and by previous court cases. It is not unrelated to the meaning from literary or art criticism, but it is specific and by no means the same. The word "derivative" also has other specialized meanings, for example in chemistry, and in mathematics.