Open-source software is an antonym for closed source and refers to any computer software whose source code is available under a license that permits users to study, change, and improve the software, and to redistribute it in modified or unmodified form. It is often developed in a public, collaborative manner.
See this Wikipedia entry for more information.
In addition, see the Open Source Initiative (OSI) web site. OSI is a non-profit corporation dedicated to managing and promoting the Open Source Definition for the good of the community, specifically through the OSI Certified Open Source Software certification mark and program.