The Collaborative International Dictionary of English v.0.48:

Aplomb \A`plomb"\, n. [F., lit. perpendicularity; ? to + plomb
   lead. See Plumb.]
   Assurance of manner or of action; self-possession.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aplomb
    n 1: great coolness and composure under strain; "keep your cool"
         [syn: aplomb, assuredness, cool, poise, sang-
         froid]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "aplomb":
   assurance, balance, balanced personality, composure, confidence,
   constancy, constraint, control, cool, coolness, discipline, ease,
   easiness, equability, equanimity, equilibrium, erectness, fastness,
   firmness, homeostasis, imperturbability, independence,
   invariability, level head, levelheadedness, nerve, nonchalance,
   orthogonality, perpendicularity, plumbness, plungingness, poise,
   possession, precipitousness, presence of mind, reliability,
   restraint, right-angledness, right-angularity, rootedness,
   sang-froid, sangfroid, savoir faire, secureness, security,
   self-assurance, self-command, self-confidence, self-conquest,
   self-control, self-denial, self-discipline, self-government,
   self-mastery, self-possession, self-restraint, sheerness, solidity,
   soundness, squareness, stability, stable equilibrium, stable state,
   steadfastness, steadiness, steady nerves, steady state, steepness,
   substantiality, undeflectability, uniformity, unshakable nerves,
   unshakableness, up-and-downness, uprightness, verticalism,
   verticality, verticalness, well-regulated mind