Free and Open Source Software
From OSGeo Wiki
Free Software licensing models and Open Source development methodologies can be abbreviated as FOSS. Some alternatives add an "L" to read FLOSS to explicitly refer to the french word "Libre" like in liberty to differentiate it to free as in gratis. Some people are irritated by the connotation to dental floss.
A range of FOSS primers, documents that can be used to convey a message to new audiences, can be found at the Open Source Network