Edit: I was not aware of the FSF's definition. I was using a definition of free software being software that you can use without having to pay for it.
To add an additional suggestion, gratis can also be used to refer to free as in free beer. Comes from a latin root and is common in spanish speaking countries to refer only to free of charge, and not as in freedom.
That’s called freeware. Also, open-source software can be paid (with the caveat that if someone buys it, you must allow them to redistribute it for free).
Depends on which "free software" definition you're referring to.
The FSF definition of "free software" requires it to be open source.