So many people ask me whether there is a distinction between "volunteerism" and "voluntarism" that I have written up my answer. Here it is:
"Voluntarism" (the older term) refers to everything voluntary. In the United States this includes, for example, religion. It certainly encompasses the entire "voluntary sector," but "voluntary" in the "voluntarism" context means not mandated by law (as government is). Many voluntary sector (nonprofit) agencies have a volunteer board because that is a legal requirement, but may not utilize volunteers in direct service in any way. There are subjects within "voluntarism" that have nothing to do with volunteers: things like UBIT legislation; proposal writing; compensation law.
"Volunteerism" was actually coined by Harriet Naylor and used for the first time in an organizational name by Ivan Scheier in the 1970's: The National Information Center for Volunteerism (NICOV). Don't let the fact that we know who invented the term deter you from taking it seriously. In the same time period someone, somewhere coined words like cyberspace, byte, nerd, and maybe 1000 others!
At any rate, "volunteerism" is a more focused term that speaks to anything relevant to volunteers and volunteering. Some people say it refers to the activity, while voluntarism speaks to the nonprofit setting. But the most important point, for me, is that "volunteerism" encompasses volunteering regardless of setting. Therefore, it allows government agencies at all levels to be included, and also covers corporate employee volunteering. Since government-related volunteering is so pervasive (think schools, libraries, parks, etc., etc.), this is not an insignificant point.
The American military confuses us even more. I once told an audience of generals at the US War College that they didn't MEAN a "Volunteer Army," they meant a "Voluntary Army," as in "non-draft." Just one more confusion in the fascinating world of volunteerism.
When we use "volunteerism," we can communicate that we are speaking about issues relevant to our work: the actions necessary to plan for, recruit, encourage, and generally support volunteers in their important efforts. So it is an important distinction and I therefore recommend that you use "volunteerism" in your work.