Linux is distributed under the GNU General Public License.
in essence, the GPL says that anyone may copy, distribute, and even sell the program, so long as changes to the source are reintroduced back to the community and the terms of the license remain unaltered. Free means that you are free to take Linux, modify it, and create your own version. Free means that you are not at the mercy of a single vendor who forces you into a kind of corporate servitude by making sure that it is extremely costly to convert to another environment. If you are unhappy with your Linux vendor or the support you are getting, you can move to the next vendor without forfeiting your investment in Linux.
In other words, "free as in speech", or simply "freedom".