“Open source” describes software whose source code is made available under a license that lets anyone use, study, change, and redistribute it. The term was adopted in 1998 and is governed by the Open Source Initiative (OSI), whose Open Source Definition sets out the criteria a license must meet. As the definition states, “open source doesn’t just mean access to the source code” - it also requires free redistribution, allowing derived works, and not discriminating against people, groups, or fields of use.
The Open Source Definition was adapted from the Debian Free Software Guidelines, themselves rooted in the earlier free-software movement. Open source and [[free-software]] cover almost exactly the same set of programs; the difference is emphasis. The free-software movement frames the issue in terms of user freedom and ethics, while “open source” was coined to stress the practical, business-friendly advantages of open development - a framing meant to appeal to companies wary of the word “free.”
The label spread quickly because it attached a clear name and a checkable definition to a way of building software that already worked: large projects like Linux, the Apache HTTP Server, and later git and GitHub showed that openly developed code could be more robust and more widely adopted than closed alternatives. Today open source underpins most of the world’s software infrastructure, from operating systems and programming-language toolchains to the libraries inside nearly every application.