Licensing
- Free software - permission for anyone to use, copy, and/or distribute, either verbatim or with modifications, either gratis or for a fee.
- Copyleft software - free software whose derivatives and copies must also be distributed as free software.
- Non-copyleft free software - Free software whose license allows derivatives to be turned into proprietary software.
- Open source software - more or less the same collection of programs as free software with some exceptions.
- Proprietary software = non-free software