Open source

In software applications, being Open Source usually means the applications' source codes are freely available to any one that wants to have them for whatever reason.

Such sources are often distributed using an OSI-approved license; usually GPL, LGPL or BSD. But other, non-OSI-approved licenses do exist, like the one used for Monkey's Audio sources.

Any programmer can freely modify such sources and release the modifications, with some catches depending on the license chosen by the copyright holder. For example, the GPL demands that modifications are released under the same license, and the Qt license forbids people of creating code branches.

More information about Open-source software => https://en.wikipedia.org/wiki/Open-source_software