The Collaborative International Dictionary of English v.0.48:

Buoyant \Buoy"ant\, a. [From Buoy, v. t. & i.]
   1. Having the quality of rising or floating in a fluid;
      tending to rise or float; as, iron is buoyant in mercury.
      "Buoyant on the flood." --Pope.
      [1913 Webster]

   2. Bearing up, as a fluid; sustaining another body by being
      specifically heavier.
      [1913 Webster]

            The water under me was buoyant.       --Dryden.
      [1913 Webster]

   3. Light-hearted; vivacious; cheerful; as, a buoyant
      disposition; buoyant spirits. -- Buoy"ant*ly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

buoyant
    adj 1: tending to float on a liquid or rise in air or gas;
           "buoyant balloons"; "buoyant balsawood boats"; "a floaty
           scarf" [syn: buoyant, floaty]
    2: characterized by liveliness and lightheartedness; "buoyant
       spirits"; "his quick wit and chirpy humor"; "looking bright
       and well and chirpy"; "a perky little widow in her 70s" [syn:
       buoyant, chirpy, perky]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "buoyant":
   adaptable, adaptive, afloat, airy, animated, blithe, bouncy,
   breezy, bright, carefree, cheerful, convalescent, corky, debonair,
   ebullient, effervescent, elastic, expansive, extensile, flexible,
   flexile, floatable, floating, floaty, free and easy, jaunty, light,
   light-hearted, lighthearted, lightsome, lively, peppy, perky,
   recuperative, recuperatory, resilient, responsive, reviviscent,
   springy, stretch, stretchable, stretchy, supernatant, vivacious,
   volatile