BookCrossing is defined as the practice of leaving a book in a public place to be picked up and read by others, who may then do likewise. The term is derived from bookcrossing.com, a free online book club which was founded to encourage the practice, aiming to "make the whole world a library."

Postcrossing is an online project for people to exchange postcards with other project members globally. The project's tag line is "send a postcard and receive a postcard back from a random person somewhere in the world!" The name Postcrossing is a union of the words postcard and crossing, and its origin "is loosely based on the Bookcrossing site".