In 1990, Judge Pierre Leval wrote his path breaking article, "Toward a Fair Use Standard," 103 Harv. L. Rev. 1111 (1990), in which he proposed the term "transformative use,":"The use must be productive and must employ the quoted matter in a different manner or for a different purpose from the original. A quotation of copyrighted material that merely repackages or republishes the original is unlikely to pass the test; in Justice Story's words, it would merely "supersede the objects" of the original. If, on the other hand, the secondary use adds value to the original -- if the quoted matter is used as raw material, transformed in the creation of new information, new aesthetics, new insights and understandings -- this is the very type of activity that the fair use doctrine intends to protect for the enrichment of society. Transformative uses may include criticizing the quoted work, exposing the character of the original author, proving a fact, or summarizing an idea argued in the original in order to defend or rebut it. They also may include parody, symbolism, aesthetic declarations, and innumerable other uses."
Nothing substitutes for Leval's article, of course, but Fairly Useful summarizes it well and opines that the article "should be seen as the doctrinal foundation fair use jurisprudence since in the" post-Campbell v. Acuff Rose era.