Originally Answered: Would contributing to Articles in Wikipedia be considered open source?
Sort of. The "source" in "open source" refers to "source code". When you write a program in a compiled computer language like C, you write code in a relatively readable way and then have a program compile it into a format that's faster for the computer to run but that can't be easily read by a human. Thus, if you want to know about how the program works or to modify it for your own uses, you want to see the source code that the compiled code was made from.
People started advocating open-source software because it gave people more freedom about how the code could be used. This is separate, however, from the freedoms and limitations of intellectual property law. Therefore, I can own copyright on my code, and sue anyone who distributes versions of my code without my permission, while simultaneously letting them read the source code. To allow anyone to also modify and redistribute my code, I have to either abandon the copyright to the public domain, or use a "free" license (that's both "free as in beer" [gratis] and "free as in speech" [libre]) that lets anyone use it, modify it, and redistribute it as they want (usually with a few conditions).
For this reason, "open-source" itself doesn't mean much nowadays, and the recommended vocabulary would probably be "FOSS", or Free and Open-Source Software.
Wikipedia isn't software, so "open-source" is meaningless unless you're using it in a very general and somewhat incorrect sense. However, Wikipedia, which uses the Creative Commons Attribution-ShareAlike 3.0 License (and for some things, also the GNU Free Documentation License 1.3), is using the sort of free licenses that let people do the "Free" part of FOSS, so there's certainly a similarity. Helping Wikipedia (such as by editing) is certainly a way of volunteering.
It should probably be noted that Wikipedia is, from a technical point of view, run using FOSS software, including the "stack" all the way from the operating system (Linux), webserver (Apache), database (MySQL), code platform (PHP), through the wiki engine itself (MediaWiki, which was started to power Wikipedia). You can contribute to the MediaWiki development process if you're familiar with PHP, and that assistance *would* be open source volunteering. :)