Free Software Foundation Europe writes:

The terminology "Open Source" refers to having access to the source code. But access to the source code is only a precondition for two of the four freedoms that define Free Software. Many people do not understand that access to the source code alone is not enough.

As an example: the finance library R-Quant, which describes itself as "open-source" but has a restrictive non-commercial license,

R-Quant is an open source project. Thus the software or other derivative materials can be downloaded and used for free for any non-commercial purpose: * Download and use binary and source distributions for free to test or evaluate the software. * Download and use binary and source distributions for free in non-commercial research, teaching or other academic activities. * This software as well as its documentation or other related materials can not be used in any commercial application without written permission or official license granted by the author.

http://www.smartquant.com/license.html