TANSTAAFL is an acronym for the adage "There Ain't No Such Thing As A Free Lunch," which was invented by science fiction writer Robert A. Heinlein and promulgated in his 1966 novel The Moon is a Harsh Mistress. Simply put, it means that you can't get something for nothing (a gift). Even if something appears to be free, there's always a catch -- someone wants something from you in return.

This argument may also be applied to natural physical processes; see thermodynamics. In mathematical finance, the term is also used as an informal synonym for the principle of no-arbitrage.