(military) a person who freely enlists for service WordNet 3.0 & Open English WordNet

A military volunteer is a person who enlists in military service by free will, and is not a conscript, mercenary, or a foreign legionnaire. Wikipedia

May refer to either non-conscripts, or to reservists Wikipedia Disambiguation

Someone who performs or offers to perform a service out of his own free will, often without payment. OmegaWiki

One who enters into military service voluntarily, but who, when in service, is subject to discipline and regulations like other soldiers; -- opposed to conscript; specifically, a voluntary member of the organized militia of a country as distinguished from the standing army. Wiktionary