The term "open source" was created by Eric Raymond (ESR)[1] on 8 February 1998 when Mozilla was in the process of making its source code public. This has been described in detail in the book 'Open Sources: Voices from the Open Source Revolution'[3].

[1] https://en.wikipedia.org/wiki/Eric_S._Raymond#Open_source

[2] http://www.catb.org/esr/open-source.html

[3] http://oreilly.com/catalog/opensources/book/raymond2.html