Proprietary software is software that does not allow its user free access to it's source code and which does not grantee its user's right to freely study, modify, remix, reproduce and redistribute the code or the binary packages produced by the code.
Proprietary software is the opposite of free software. Because the user cannot inspect the workings of the program, proprietary software is an inherent privacy risk for the user, because it is next to impossible to verify developer's claims without advanced knowledge of reverse engineering. Thus, any security and privacy claims made by the developer have to be taken purely at blind faith.
Here at PrivacyWiki, we actively discourage use of proprietary software and services where a viable free software alternative of comparable quality exists.
However, we are also not free software absolutists, and we understand that sometimes usage of proprietary software cannot be avoided and that proprietary software can also be privacy preserving.