The Collaborative International Dictionary of English v.0.48:

Musketeer \Mus`ket*eer"\, n. [F. mousquetaire; cf. It.
   moschettiere.]
   A soldier armed with a musket.
   [1913 Webster]

WordNet (r) 3.0 (2006):

musketeer
    n 1: a foot soldier armed with a musket

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "musketeer":
   Nimrod, Zouave, archer, artilleryman, bersagliere, bowman,
   cannoneer, carabineer, chasseur, crack shot, dead shot, deadeye,
   dogface, doughfoot, expert rifleman, foot soldier, footslogger,
   fusileer, good shot, grenadier, grunt, gun, gunman, gunner, hunter,
   infantryman, light infantryman, marksman, markswoman, paddlefoot,
   rifle, rifleman, sharpshooter, shooter, shot, sniper,
   targetshooter, toxophilite, trapshooter