1. Generically, open source refers to a program in which the source code is available to the general public for use and/or modification from its original design free of charge, i.e., open.
2. Open source software is software with source code that anyone can inspect, modify, and enhance.
http://www.webopedia.com/TERM/O/open_source.html (1.); https://opensource.com/resources/what-open-source (2.)