What is "proprietary software" or "closed source software"?

"Proprietary" software is used as an antonym for open source software. It therefore identifies conventionally licensed software.  Although strictly speaking the term "proprietary" is inaccurate since open source software has one or more "owners" who are “proprietary,” i.e., the holders of rights), it is commonly used in practice.  "Closed source" is of limited use as an antonym since open source code is only one of several criteria that defines open-source software (→ What is open source software?).  Even software offered for free such as freeware can be "proprietary" because the licensee cannot acquire the same freedom of use as with open-source software.
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