Ultimately whether one work is derivative of another is a question of fact, and there is no absolute rule beyond taking it to a court. Many cases are clear. A Full translation is obviously a derivative work,
The definition in 17 USC 101 is:
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”.
A mere description of a work, such as a one-paragraph summary, is probably not a derivative work, but as the summary gets longer and fuller, it starts to approach an "abridgment or condensation". One test is if the summery could reasonably replace the original, at least for some consumers. But there is no clear cut place to draw the line.
By the way, If something is a derivative work, it is infringing provided only that the original is in copyright and the creator of the derivative did not have permission from, the copyright holder on the original. Fair use may prevent an adverse judgement, but is still an infringement.
As the question referred to fair use, a strictly US legal concept, I am assuming US law here.
