Open source software is software whose source code is publicly available for anyone to view, use, modify, and redistribute, usually under a license that sets the terms for doing so.
"Source code" is the human-readable instructions a program is built from. Most commercial software only ships the compiled, unreadable version — but open source projects publish the actual code, so anyone can inspect how it works, fix bugs, or build on top of it.
Open source underpins a huge amount of modern software — the Linux operating system, the Chromium browser engine, most programming languages, and countless libraries. It benefits from many people reviewing and improving the same code, and it lets developers avoid rebuilding common infrastructure from scratch. "Open source" is about the license and code access — it doesn't automatically mean "free," "unpaid," or "unsupported," though a lot of it is free to use.
Last reviewed: September 2026