According to Richard Stallman
and the Free Software Foundation
, free software is software that everyone is free to copy, redistribute and modify.
That implies it must be available as source code.
It does not imply that it is free of charge, so anyone can sell free software so long as they don't impose any new restrictions on its redistribution or use.
is free in this sense, though it is not really software
There are many other kinds of "free software" in the more obvious sense of "free of charge".