Any person who forfeits their personal freedom to join the ranks of any government organization (esp. one guilty of tyranny) with aspirations to better their lives, sometimes possessing great desire to become a trained killer, possibly also harboring a death wish. When the entire word is capitalized out of context this may refer to the popular video game, Final Fantasy VII, in which the protagonist Cloud joins SOLDIER of Shinra to become like his idol and rival Sephiroth.
"Don't go thinkin' you all bad jus' cause you was in SOLDIER!" - Barret Wallace, Final Fantasy VII character, and leader of the rebel faction AVALANCHE.