I think Alex's definition is a little off of the commonly understood use. Mostly it refers to the National Guard and the Army Reserve as well as the reserves of other branches of the armed forces, people who spend most of their time as civilians but who spend some time, as Jackie pointed out, in some service related business. Perhaps Alex's definition was common in other times, such as WWII, when many people joined the army for the duration with no intent on making the military a career. Nowadays, many people will join the military for various benefits that were not available to them otherwise, for example, paying for college education. As long as they are full-time regular army they are not called citizen-soldiers.