The Collaborative International Dictionary of English v.0.48:

Subaltern \Sub*al"tern\, a. [F. subalterne, LL. subalternus, fr.
   L. sub under + alter the one, the other of two. See Alter.]
   1. Ranked or ranged below; subordinate; inferior;
      specifically (Mil.), ranking as a junior officer; being
      below the rank of captain; as, a subaltern officer.
      [1913 Webster]

   2. (Logic) Asserting only a part of what is asserted in a
      related proposition.
      [1913 Webster]

   Subaltern genus. (Logic) See under Genus.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Subaltern \Sub*al"tern\, n.
   1. A person holding a subordinate position; specifically, a
      commissioned military officer below the rank of captain.
      [1913 Webster]

   2. (Logic) A subaltern proposition. --Whately.
      [1913 Webster]

WordNet (r) 3.0 (2006):

subaltern
    adj 1: inferior in rank or status; "the junior faculty"; "a
           lowly corporal"; "petty officialdom"; "a subordinate
           functionary" [syn: junior-grade, lower-ranking,
           lowly, petty(a), secondary, subaltern]
    n 1: a British commissioned army officer below the rank of
         captain

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "subaltern":
   ADC, CO, OD, aide, aide-de-camp, brigadier, brigadier general,
   captain, chicken colonel, chief of staff, cog, colonel, commandant,
   commander, commander in chief, commanding officer,
   commissioned officer, common, commonality, commonalty,
   company officer, creature, demeaning, disadvantaged, exec,
   executive officer, field marshal, field officer, first lieutenant,
   five-star general, flunky, follower, four-star general, general,
   general officer, generalissimo, hoi polloi, humble, in the shade,
   inferior, infra dig, jemadar, junior, junior officer, less, lesser,
   lieutenant, lieutenant colonel, lieutenant general, lightweight,
   low, lower, lower class, lower orders, lowly, major, major general,
   marechal, marshal, masses, minor, modest, officer,
   one-star general, orderly officer, ordinary, pawn, poor relation,
   risaldar, scrub, second fiddle, second rank, second string,
   secondary, senior officer, servile, shavetail, sirdar,
   staff officer, sub, subahdar, subject, sublieutenant, subordinate,
   subservient, the Old Man, the brass, third rank, third string,
   third stringer, three-star general, top brass, two-star general,
   underling, underprivileged, understrapper, vulgar, yes-man