The open source definition has a non-discrimination clause. From the annotated Open Source Definition:
5. No Discrimination Against Persons or Groups
The license must not discriminate against any person or group of persons.
Rationale: In order to get the maximum benefit from the process, the maximum diversity of persons and groups should be equally eligible to contribute to open sources. Therefore we forbid any open-source license from locking anybody out of the process.
Some countries, including the United States, have export restrictions for certain types of software. An OSD-conformant license may warn licensees of applicable restrictions and remind them that they are obliged to obey the law; however, it may not incorporate such restrictions itself.
This clearly means that for a license to be considered open source, the license must not discriminate against any person or group. The license may not, for example, only allow non-profits to use the work (because non-profits are good), or disallow clowns to use the work (because clowns are creepy).
I heard people say that this also means I may not selectively license a work that I hold the copyright of under a free license. Is that true?
For example, can I grant 501(c) organizations an GPL-3 license to my software, but nobody else?
It is clear that according to the terms, they are allowed to re-distribute the software to anyone else, so the effectiveness of this might be low, but my question only pertains to whether or not I have the right to deny an open source license to some people/groups.