Usually a synonym for ClosedSource, it means the opposite of open or Free.

Do not confuse Proprietary software 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..

• almost always charged for
• often only available to specific customers
• almost always forbids any modification at all, even for in-house use of the customer only.

Proprietary software leaves you at the mercy of its vendor regarding support and the pricing thereof, availability and cost of upgrades and so on.

Of course, not only software can be Free, hence the opposite also applies more generally. MicrosoftOffice file formats, for example, do not follow open standards and are undocumented; thus, they too are Proprietary.

Hackers generally sneer at anything Proprietary.