An open source license is a form of computer software copyrighting in which the author allows the source codes to be accessed, used and manipulated by anyone without fear of reprisal for copyright infringement. This means that anyone who downloads the source code can make any desired modification to the source code. This allows users to fully customize a piece of software to fit their needs.

A modified copy of a piece of software that has an open source license may or may not be allowed to be distributed legally. An author may stipulate in his or her source code that distribution of any modification may be an infringement of the copyright and is expressly forbidden. If this is the case, modified content can be used only by the modifier, and he or she cannot share the changes with anyone else.

The term "open source software" does not mean a piece of software is free, although most open source software is distributed that way. Most authors who provide open source software free of charge specify that any modification made to the software cannot be sold, though it may be distributed free of charge. Any attempt to sell open source software modifications can result in legal and civil actions. Some open source authors ask for voluntary donations upon download of their programs, though the programs are not technically sold.

The author of software with an open source license may control the way modifications are distributed by requesting that modifiers add specific information to the source code. There are four distinct ways that they can request control of modified source codes. The first is called transparent, in which the modifying author must keep the original code and all modifications transparent so everyone can see what has been done.

The second request is known as recombinant. In this request, the author only allows modifiers to edit or rearrange existing code in the source code, but they are forbidden from adding their own additional codes. The third request is called credited, and the author simply requests that his or her work is credited as being the original and any subsequent modifications are labeled with credits for the person who made the modifications. The last request that the author can make is known as circulating, and he asks that the original software with the open source license remain downloadable in an unmodified state and labeled as the original.