What is Open Source Software?

A software product which is licensed to permit modifications and redistribution of its source code.(http://en.wiktionary.org/wiki/open-source).

Open source software (OSS) is defined as computer software for which the source code and certain other rights normally reserved for copyright holders are provided under a software license that meets the Open Source Definition or that is in the public domain. This permits users to use, change, and improve the software, and to redistribute it in modified or unmodified forms. It is very often developed in a public, collaborative manner. Open source software is the most prominent example of open source development and often compared to user-generated content.[1] The term open source software originated as part of a marketing campaign for free software.[2] A report by Standish Group states that adoption of open source software models has resulted in savings of about $60 billion per year to consumers.[3][4]

http://en.wikipedia.org/wiki/Open_source_software