Open source software is software, including source code, that may be freely modified and distributed. The term 'open source' represents a variety of requirements to which such software must adhere. For additional research, see The Open Source Initiative.
Excerpt from the Mission statement of the Open Source Initiative:
"Open source is a development method for software that harnesses the power of distributed peer review and transparency of process. The promise of open source is better quality, higher reliability, more flexibility, lower cost, and an end to predatory vendor lock-in."