Definition, Library of Congress Copyright Office Website [PDF}:
In determining whether the use made of a work in any particular case is a fair use, the factors to be considered shall include the:
  1. Purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
  2. Nature of the copyrighted work;
  3. Amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the
  4. Effect of the use upon the potential market for or value of the copyrighted work.