"Open Source Software" describes computer programs which are circulated under a so-called "open source license". In this context "Open Source" not only means the free access to the source code, but also the inclusion of other criteria, under which open source software may be divulged. Only if the computer programs and their licenses respectively, under which the computer programs may be distributed, comply with the criteria set up by the Open Source Initiative, Inc. ("OSI") in their ("Open Source Definition") the OSI includes such license in their overview and thereby approves the conformity of said license with the "Open Source Definition". The OSI describes its "License Approval Procedure" in detail. Up to today the OSI has approved approximately 60 open source licenses as complying with their given definition.

Among other issues open source softeware ("OSS") licenses basically allow the free redistribution and copying of the software, the free access to the source code and the modification as well as derived works. The free access to the source code is technically necessary for the further development of the software. The mentioned liberties concerning the use of the software contained in the license are legal premises for the possibility of further development of such software.

In practice OSS licenses are distinguished according to the criteria "Copyleft/Non-copyleft" as well as whether they are GPL-compatible or GPL-incompatible (see: License Categories).