Open Source

The definition according to the Open Software Initiative is “Software where the source code (the language in which the program is written) is freely distributed with the right to modify the code, and on the condition redistribution is not restricted, and is obtainable for no more than the reasonable cost of reproduction.” As opposed to proprietary software that provides you with executable binary code only, not the human readable source code it is derived from.
Source Forge

also see: Open Source and The Free Software Foundation Apple Open Source