From The Jargon File (version 4.4.7, 29 Dec 2003) :
free software
n.
As defined by Richard M. Stallman and used by the Free Software movement,
this means software that gives users enough freedom to be used by the free
software community. Specifically, users must be free to modify the software
for their private use, and free to redistribute it either with or without
modifications, either commercially or noncommercially, either gratis or
charging a distribution fee. Free software has existed since the dawn of
computing; Free Software as a movement began in 1984 with the GNU Project.
RMS observes that the English word ?free? can refer either to liberty
(where it means the same as the Spanish or French ?libre?) or to price
(where it means the same as the Spanish ?gratis? or French ?gratuit?). RMS
and other people associated with the FSF like to explain the word ?free? in
?free software? by saying ?Free as in speech, not as in beer.?
See also open source. Hard-core proponents of the term ?free software?
sometimes reject this newer term, claiming that the style of argument
associated with it ignores or downplays the moral imperative at the heart
of free software.