What distinguishes open-source software from proprietary software?

The defining difference between open-source and proprietary software lies in the rights granted over the software's source code. Open-source software is distributed with its source code freely available, and its licence explicitly allows users to study, modify, and redistribute that code, including their own modified versions. Proprietary software keeps its source code hidden and its licence restricts users to running the program as supplied, without any legal right to alter or redistribute it.

Cost is not the defining distinction: some open-source software is distributed without charge, but the licence terms concerning modification and redistribution are what technically define the category, not the price. Security is also not a reliable distinguishing feature, since either type of software can be well written or poorly written, and openness of the source code does not by itself guarantee stronger security. The characteristic that consistently separates the two categories is that open-source licensing permits modification and redistribution of the code, while proprietary licensing does not.

Examination reminder: when a question asks what technically "distinguishes" open-source from proprietary software, look for the answer built around rights to modify and redistribute the source code, rather than price or security claims.