Open Source: When programmers can read, redistribute, and modify the source code for a piece of software.

Free Software: Free software is a matter of liberty, not price. Users of free software are free to run, copy, distribute, study, change and improve the software.

Proprietary Software: In the context of Free and Open Source Software (FOSS), proprietary software is software that is not licensed under a FOSS license.

Commercial Software: Software that is sold. FOSS software can also be commercial.