Fair use is a legal concept which allows copyrighted work to be published in a different context without first getting the copyright holder's permission. In the United States, the fair-use law applies to all copyrighted work, regardless of medium, format or publication status. Fair-use and copyright law vary by country.US Copyright Office: Fair Use

Fair use is determined on a case-by-case basis, per guidelines written into 17 USC § 107, or "Limitations on Exclusive Rights: Fair Use." The law outlines several key factors to be considered when determining if copying complies with fair use:Cornell University: Copyright law: Fair use

1. The purpose and character of the use;
2. The nature of the use;
3. The proportion used as it relates to the copyrighted work as a whole; and
4. The effect of such use on the potential market-value of the copyrighted work.