BookCrossing (also: BC, BCing or BXing) is defined as "the practice of leaving a book in a public place to be picked up and read by others, who then do likewise." The term comes from bookcrossing.com, a free online book club which began in order to further the practice, with the goal to "make the whole world a library."

The 'crossing' or exchanging of books may take many different forms, including wild releasing books in public, direct swaps with other members of the website, or "book rings" in which books travel in a set order to people who want to read that book. The community aspect of BookCrossing.com has grown in ways that were not expected at the start, in the form of blog or forum discussions, mailing lists and yearly conventions all over the world.