Fair use is the right to use a copyrighted work under certain conditions without permission of the copyright owner.

The doctrine helps prevent a rigid application of copyright law that
would stifle the very creativity the law is designed to foster.

Fair use of copyrighted works, as stated in US copyright law, “for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright.”

Transformative use is a relatively new addition to fair use law, having been first raised in a Supreme Court decision in 1994. (Campbell v. Acuff-Rose Music, 510 U.S. 569 (1994.) A new work based on an old one work is transformative if it uses the source work in completely new or unexpected ways.

Transformative uses
are those that add something new, with a further purpose or different
character, and do not substitute for the original use of the work.

Transformativeness is a characteristic of such derivative works that makes them transcend, or place in a new light, the underlying works on which they are based. In computer- and Internet-related works, the transformative characteristic of the later work is often that it provides the public with a benefit not previously available to it, which would otherwise remain unavailable.

Such transformativeness weighs
heavily in a fair use analysis and may excuse what seems a clear copyright infringement from liability.

Supreme Court case Campbell v. Acuff-Rose Music is important in large part because it ordered that commerciality should be given less weight in fair-use determinations and transformation greater weight.