The DoD's Definition of Open Source Software
As defined by the Department of Defense, "Open Source Software (OSS) is software for which the human-readable source code is available for use, study, reuse, modification, enhancement, and redistribution by the users of that software. In other words, OSS is software for which the source code is 'open'."1>
It’s a Licensing Structure
A distinguishing mark of OSS is its licensing structure. There are a wide variety of open source licenses2 recognized by the Open Source Initiative (OSI). The purpose of all OSS licenses is to enable the distribution of a project's source code. Some of the OSS licenses are very permissive while others are more restrictive. Whatever the license, they share the common perspective of protecting the intent of a original project's owner. As projects grow or fork, it is common to modify or absorb previous licenses—most licenses are easily accommodate this growth.
It’s a Philosophy
The most common philosophical position OSS developers and advocates take is that Open Source empowers innovation. This view believes in the innovative power of people, especially in large numbers; it also assumes that what will benefit one group will likely benefit other groups. Open Source as a philosophy provides a sound economical development environment by creating communities of interest and empowering them to grow that community.
A more extreme subset of the Open Source as a philosophy, Free/Libre Open Source Software (FLOSS), views software as an idea which should be freely available. Similar to the way books contain ideas which are freely available to be studied and shared, the Free Software Movement (or Software Libre) considers source code an idea that should be available to be studied and shared freely. The goal of FLOSS philosophy is not eliminate development cost, but to free ideas that drive innovation. Richard Stallman explained, "free software is a matter of liberty, not price. To understand the concept, you should think of 'free' as in 'free speech', not as in 'free beer'."
It’s Community & Social Movement
A natural outgrowth of the Open Source philosophy is the spontaneous development of widely varied communities of interest that collectively make up the Open Source Community. Many consider this grassroots growth of Open Source communities a social movement. Although the "social movement" classification may be an overstatement, the fact that there is an active community of Open Source developers in undeniable.
It’s a Practical Solution to Common IT Problems
Redundant problems should never be met with redundant problem solving3. Open Source provides a practical solution to common problems that recur throughout the Department of Defense and the Federal Government. Once a problem is identified and a software solution has been created under an Open Source license, the solution can be shared and re-implemented rather than having to be re-invented.
1 DoD Policy Memo: Clarifying Guidance Regarding OSS(2009)
2 Open Source Initiative - opensource.org/licenses/alphabetical
3 Open Source Efficiencies and Obstacles in the Department of Defense, 2010