A synonym for ClosedSource.
Do not confuse this with commercial software. A company can be making money from Free software, which is thus commercial. On the other hand, non-commercial software may still be Proprietary: this is called freeware (not to be confused with FreeSoftware).
Proprietary software is distributed in BinaryExecutable form under a restrictive license. Where SourceCode is available, if at all, it is..
Proprietary software leaves you at the mercy of its vendor regarding support and the pricing thereof, availability and cost of upgrades and so on.
Hackers generally sneer at Proprietary software.
Non-software can be Proprietary. MicrosoftOffice file formats, for example, are Proprietary.