Three definitions of open source
Open Source as Legal Innovation: the creation of licenses that enabled source code to be open, including copy left which forced contribution of changes.1
Open Source as Collaboration Method: the process of working out in the open with a distributed group of contributors jointly making changes to a central code base.2
Open Source as Ethos: a belief that code should be open for inspection and free as in freedom. That code that runs applications should be available and changeable. That many eyes make all bugs shallow.