The concept that ideas cannot be copyrighted, but their particular expression can.
The idea / expression dichotomy is fundamental in copyright law. For example, the particular text of Stephenie Meyer’s “Twilight” series of vampire novels is protected by copyright, but the idea of a girl falling in love with a vampire cannot be protected.
While this may seem obvious or self-evident, the line between the two is not always so easy to find, and aggressive rights-holders continue to try to push the limits of to what they can claim copyright. For example, in the U.S. case Metro-Goldwyn-Mayer, Inc. v. American Honda Motor Co., Inc., the holder of the rights to the intellectual property making up the James Bond character successfully sued an automobile company for an advertisement they had aired. MGM claimed that the ad’s content was sufficiently similar to or evocative of, James Bond, that it had infringed, although no actual copying took place.
'In U.S. law, idea / expression is usually held up, along with fair use, as a “safety valve” that prevents the monopolies granted by copyright from interfering with public policy, freedom of speech, and more.
Under certain circumstances, courts have held that there are a limited number of ways in which to express a particular idea (such as the rules for lotteries or sweepstakes) and that therefore, no copyright can be held in those materials. This is known as the “merger” doctrine.