In general, it is any program whose source code is made available for use or modification by users, developers, or hackers. Historically, the makers of proprietary software have generally not made source code available.
Open source software is usually developed as a public collaboration and made freely available. For example, in an effort to stay viable in its browser competition with Microsoft (prior to its acquisition by AOL), Netscape made its browser source code (Mozilla) freely available, encouraging users to improve it.
The open source movement has gained momentum as commercial enterprises have begun to consider Linux as an open alternative to Windowsoperating systems.
Open source doesn't just mean access to the source code, however. The distribution terms of open source software must comply with the following criteria:
distribution in source code as well as compiled form,
distribution of modifications and derived works under the same licensing terms as those of the original software,
preservation of the original author's source code,
no discrimination against persons or groups,
no discrimination against fields of endeavor,
licensing so that the rights attached to the program apply to all users,
licensing that is not specific to use in a particular product,
licensing that does not contaminate other software.
Open Source, with capital letters, is a certification mark owned by the Open Source Initiative (see OSI).