The Cathedral and the Bizarre

In 1998 Eric S. Raymond published an epochal game-changing book called The Cathedral and the Bazaar . in it he laid the foundations, methodology and aims for the open source movement. 

Before Raymond, the phrase 'open source' had a definite meaning in computing which is quite different from the sense it has now. In 1990 when you said a program was 'open source', you meant that you could read the source code; the actual code the person had written to create the program. Closed source programs did not give you that ability. Beyond that, there were no requirements on open source. In theory one could charge for open source code or place restrictions on it's use.

Raymond meant by 'open source', software licensed under liberal licenses like BSD and MIT in which the author not only made the software readable, but also effectively relinquished any creative or financial control over his creation.  Raymond's usage of the phrase was so influential that it has now transplanted the original usage.

The freeing of code from license restrictions was integral to a new methodology for developing programs.  Raymond believed that this methodology would be so effective that it would sweep non-open or closed source programs out of the market place and change the nature of society.  The methodology required that code could be freely shared between programmers through the new technology of the Internet.  Hundreds of programmers working all over the world would come together to create software collaboratively.