Of software: having the source code freely available for possible modification and redistribution
Merriam-Webster.
Example: Linux, MySQL, Ruby on Rails, and WordPress are examples of open source software.
About Open Source
Open source really began with products giving free access to design. With the advent of the internet, the term evolved to refer widely to computer programs offering complete source code with the intent for public collaboration and improvement.
As an open source platform, WordPress allows for a large community of contributors to develop tools, plugins, features, and fixes which allow for many features and a high level of usability and security. The large, active pool of developers and WordPress experts add to a network of support to turn to when users encounter difficulties.
ClarkWP WordPress Magazine 