The Collaborative International Dictionary of English v.0.48:

Commandant \Com`man*dant"\, n. [F., orig. p. pr. of commander.]
   A commander; the commanding officer of a place, or of a body
   of men; as, the commandant of a navy-yard.
   [1913 Webster]

WordNet (r) 3.0 (2006):

commandant
    n 1: an officer in command of a military unit [syn: commanding
         officer, commandant, commander]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "commandant":
   ADC, CO, OD, aide, aide-de-camp, brigadier, brigadier general,
   bull, captain, castellan, chatelain, chatelaine, chicken colonel,
   chief of staff, colonel, commander, commander in chief,
   commanding officer, commissioned officer, company officer,
   custodian, director, exec, executive, executive officer,
   field marshal, field officer, first lieutenant, five-star general,
   four-star general, gaoler, general, general officer, generalissimo,
   governor, guard, guardian, intendant, jailer, jemadar,
   junior officer, keeper, lieutenant, lieutenant colonel,
   lieutenant general, major, major general, manager, marechal,
   marshal, master, officer, one-star general, orderly officer,
   principal keeper, prison guard, risaldar, ruler, screw,
   senior officer, shavetail, sirdar, staff officer, subahdar,
   subaltern, sublieutenant, the Old Man, the brass,
   three-star general, top brass, turnkey, two-star general, warden,
   warder