“For the purposes of copyright law, the nub of the definitions [of parody] ... is the use of some elements of a prior author's composition to create a new one that, at least in part, comments on that author's works.” Campbell v. Acuff-Rose Music, 510 U.S. 569, 580 (1994).
Parody – Example
Campbell v. Acuff-Rose Music, 510 U.S. 569 (1994) - 2 Live Crew created a hip-hop song based on Roy Orbison's rock ballad "Oh, Pretty Woman." The new hip-hop song was found to be a parody because it used elements of the prior work to comment on the original.
Not Parody – Example
Dr. Seuss Enterprises, L.P. v. ComicMix LLC, 256 F.Supp.3d 1099 (S.D. Cal. 2017) - Defendants created a book called "Oh, the Places You’ll Boldly Go!," which combined aspects of Dr. Seuss’s works with elements from Star Trek. The book was found to not be a parody, but rather a “mash-up” that combined Dr. Seuss's style with Star Trek’s tropes.