The Open Source Definition is a document published by the Open Source Initiative, to determine whether a software license can be labeled with the open-source certification mark.
The definition was taken from the exact text of the Debian Free Software Guidelines, written and adapted primarily by Bruce Perens with input from the Debian developers on a private Debian mailing list. The document was created 9 months before the formation of 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:
individual technology or style of interface.
The open source movement's definition of open source software by the Open Source Initiative and the official definitions of free software by the Free Software Foundation (FSF) basically refer to the same software licenses (with a few minor exceptions see Comparison of free and open-source software licenses), both definitions stand therefore for the same qualities and values. Despite that, FSF founder Richard Stallman stresses underlying philosophical differences when he comments:
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.— Free Software Foundation
Open Knowledge International (OKI) described in their Open Definition for open content, open data, and open licenses, "open/free" as synonymous in the definitions of open/free in the Open Source Definition, the FSF and the Definition of Free Cultural Works:
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.— The Open Definition
Prior to 1998, Free Software referred either to the Free Software Foundation (and the watchful, micromanaging eye of Stallman) or to one of thousands of different commercial, avocational, or university-research projects, Processes, licenses, and ideologies that had a variety of names: sourceware, freeware, shareware, open software, public domain software, and so on. The term Open Source, by contrast, sought to encompass them all in one movement.