The Open Source Definition

Open-source certification document published by the Open Source Initiative

Open source doesn't just mean access to the source code. The distribution terms of open-source software must comply with the following criteria:

The term “open source” software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licences that we consider too restrictive, and there are free software licences they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free.

This essential meaning matches that of "open" with respect to software as in the Open Source Definition and is synonymous with “free” or “libre” as in the Free Software Definition and Definition of Free Cultural Works.