1. In marketroid-speak, superior; implies a product imbued with exclusive magic by the unmatched brilliance of the company's own hardware or software designers.
2. In the language of hackers and users, inferior; implies a product not conforming to open-systems standards, and thus one that puts the customer at the mercy of a vendor able to gouge freely on service and upgrade charges after the initial sale has locked the customer in. Often used in the phrase “proprietary crap”.
3. Synonym for closed-source or non-free, e.g. software issued without license rights permitting the public to independently review, develop and redistribute it.
Proprietary software should be distinguished from commercial software. It is possible for software to be commercial (that is, intended to make a profit for the producers) without being proprietary. The reverse is also possible, for example in binary-only freeware.