Proprietary software, coined by the free software community, is a term for computer software with restrictions on use or private modification, or with restrictions judged to be excessive on copying or publishing of modified or unmodified versions. These restrictions are placed on it by one of its proprietors. Similarly, closed source is a term for software whose licence does not meet the Open Source Definition.

These restrictions are enforced by either legal or technical means, or both. The most common form of technical restriction is by releasing only computer-readable programs to users and withholding the human-readable source code. Means of legal enforcement can involve copyright (with a restrictive software licence) and patents. The source code of such programs is usually regarded as a trade secret by the owner. Access to source code by third parties commonly requires the party to sign a non-disclosure agreement.