Open source software (OSS) can be defined as computer software for which the human-readable source code is made available under a copyright license (or arrangement such as the public domain) that meets the Open Source Definition. This permits users to use, change, and improve the software, and to redistribute it in modified or unmodified form. It is very often developed in a public, collaborative manner.

Open Source Initiative

Free software or software libre is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with minimal restrictions only to ensure that further recipients can also do these things. In practice, for software to be distributed as free software, the human readable form of the program (the "source code") must be made available to the recipient along with a notice granting the above permissions. Such a notice is a "free software licence", or, in theory, could be a notice saying that the source code is released into the public domain.

Free Software Definition

Free Software Foundation

OSI and FSF agree on all widely used free software licences. OSI's list is different from FSF's list because the two organisations have reviewed different sets of licences. There are a few licences that OSI have approved that the FSF has not, and vice versa, but these are licences that are used by niche projects or none at all.

Dual-license model, where a code base is published under a traditional open source license and a commercial license simultaneously. Vendors typically charge a perpetual license fee for additional closed-source features, supplementary documentation, testing, and quality, as well as intellectual property indemnification to protect the purchaser from legal liability.