Open source software is software that can be freely used, changed, and shared (in modified or unmodified form) by anyone. Open source software is made by many people, and distributed under licenses that comply with the Open Source Definition.
The Open Source Initiative (OSI) is a global non-profit that supports and promotes the open source movement. Among other things, we maintain the Open Source Definition, and a list of licenses that comply with that definition. See our about and history pages for more.