Software that everyone is free to copy, redistribute and modify. That implies free software must be available as source code, hence "free open source software" - "FOSS". It is usually also free of charge, though anyone can sell free software so long as they don't impose any new restrictions on its redistribution or use. The widespread acceptance of this definition and free software itself owes a great deal to Richard Stallman and the Free Software Foundation.
There are many other kinds of "free software" in the sense of "free of charge". See "-ware".
This dictionary is free in both senses, though since it is documentation not software it is distributed under the GFDL.