Open source

Open source is a method and philosophy for software licensing and distribution designed to encourage use and improvement of software written by volunteers by ensuring that anyone can copy the source code and modify it freely.

Description
The term "open source" is now more widely used than the earlier term "free software" (promoted by the Free Software Foundation) but has broadly the same meaning - free of distribution restrictions, not necessarily free of charge. There are various open source licenses available, such as the GNU General Public License. Programmers can choose an appropriate license to use when distributing their programs.

Apple presently utilizes open source methods for the development of portions of its software that it does not protect as trade secrets, such as the WebKit browser engine used by Safari and the XNU kernel of macOS and iOS.