The idea that once the first legitimate sale of a physical embodyment of a copyrighted work has taken place, the copyright holder has no claim to control further sales or many uses of the particular copy.
The first sale doctrine is a concept found in U.S. copyright law, and in some form in some other jurisdictions where it may be known as ”exhaustion of rights.” For example, if a person buys a book (a physical paper copy), that person can resell the book without the permission of the rights-holder.
The first sale doctrine has become more important with the advent of non-rivalrous digital goods, goods that can be copied and shared without transfers of possession. The question of what it means to “own” something is now more difficult to answer. Many software companies and other purveyors of digital goods have attempted to handle this by saying that users are actually purchasing a license to use, rather than buying an actual “thing.” This distinction is often lost on users, though, who are frequently baffled and frustrated when they cannot do things they assumed they could with something that, in their minds, they own.