The Open Source Definition (OSD) is a policy document published by the Open Source Initiative in 1998. Derived from the Debian Free Software Guidelines written by Bruce Perens, the definition is the most common standard for open-source software. The definition has ten criteria, such as requiring freely accessed source code and granting the open-source rights to everyone who receives a copy of the program. Covering both copyleft and permissive licenses, it is effectively identical to the definition of free software, but motivated by more pragmatic and business-friendly considerations. The Open Source Initiative's board votes on proposals of licenses to certify that they are compliant with the definition, and maintains a list of compliant licenses on its website. The definition has been adapted into the Open Knowledge Foundation's Open Definition for open knowledge and into open hardware definitions.

There have been several attempts to define open source and free software. Amongst the earliest was Free Software Foundation's Free Software Definition, which then defined as the three freedoms of Free Software (Freedom Zero was added later). Published versions of FSF's Free Software Definition existed as early as 1986, having been published in the first edition of the (now defunct) GNU's Bulletin.

The Debian Free Software Guidelines (DFSG) was first published together with the first version of the Debian Social Contract in July 1997. The primary author was Bruce Perens, with input from the Debian developers during a month-long discussion on a private mailing list, as part of the larger Debian Social Contract. Perens was copied to an email discussion between Ean Schuessler (then of Debian) and Donnie Barnes of Red Hat, in which Schuessler accused Red Hat of never elucidating its social contract with the Linux community. Perens realized that Debian did not have any formal social contract either, and immediately started creating one. The (then) Three Freedoms, which preceded the drafting and promulgation of the DFSG, were unknown to its authors.

The guidelines were:

As Netscape released the open-source Mozilla browser in 1998, Bruce Perens again drafted a set of open-source guidelines to go with the release. It has been claimed that the Open Source Definition was created by re-titling the exact text of the DFSG.

A modified version of this definition was adopted by the Open Source Initiative (OSI) as the Open Source Definition. The OSI uses the label "open source", rather than "free software", because it felt that the latter term had undesirable ideological and political freight, and it wanted to focus on the pragmatic and business-friendly arguments for open-source software. It adopted a closed rather than membership-driven organizational model in order to draft the definition and work together with a wider variety of stakeholders than other free or open-source projects.

Once the DFSG became the Open Source Definition, Richard Stallman saw the need to differentiate free software from open source and promoted the Free Software Definition.

In November 1998, Ian Jackson and others proposed several changes in a draft versioned 1.4, but the changes were never made official. Jackson stated that the problems were "loose wording" and the patch clause.