Software can be classified as "open source" if the source code is made available to its users and they have the freedom to modify and distribute it and its derivatives. In general terms, users are not subject to restrictive End-User Licence Agreements (EULAs) that must be accepted before using traditional proprietary software.

For a more precise definition, please have a look at The Open Source Definition.

Other terms for such software, some more specific than others, include "OSS" (open-source software), "FOSS" (free & open-source software), and "FLOSS" (free/libre open-source software). As "FLOSS" is the most specific, it will be used interchangeably with "open source" throughout this document.

FLOSS promotes software reliability and quality by supporting independent peer review and rapid evolution of source code, the programming that defines the software.