Fair Use is a provision within US Copyright Law (US Code Title 17, Section 107) that allows for certain uses of copyrighted material without the permission of, or payment to, the rights holder. Fair Use is applied in situations where the benefit of a given use exceeds the economic harm to the copyright owner. The statute names "purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research" as favoring fair use, but other purposes, including those invoked by for-profit companies such as Google and the publisher Dorling Kindersley have also been found to qualify for fair use.
US Copyright law outlines four factors that must be evaluated to determine if a given use is "Fair:"
No one factor is to be overlooked in favor of the others; rather, they are to be evaluated in combination. If an evaluation determines that a use is likely not Fair, it is best to look to other limitations and exceptions with copyright law that might cover the use or, failing that, ask permission from the copyright owner.