Free software, as defined by the Free Software Foundation, is software which can be used, copied, studied, modified and redistributed without restriction. Freedom from such restrictions is central to the concept, with the opposite of free software being proprietary software and not software which is sold for profit, commercial software. The usual way for software to be distributed as free software is for the software to be accompanied by a free software license (or be in the public domain), and the source code of the software to be made available (for a compiled language).