WordNet (r) 3.0 (2006):

weightlessness
    n 1: the property of being comparatively small in weight; "the
         lightness of balsa wood" [syn: lightness,
         weightlessness] [ant: heaviness, weightiness]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "weightlessness":
   airiness, ascent, asteroids, aurora particles, blackout,
   bubbliness, buoyancy, cosmic particles, cosmic ray bombardment,
   daintiness, delicacy, downiness, ethereality, floatability,
   fluffiness, foaminess, frothiness, gentleness, gossameriness,
   intergalactic matter, lack of weight, levitation, levity,
   lightness, meteor dust impacts, meteors, radiation, softness,
   space bullets, tenderness, the bends, unheaviness, volatility,
   yeastiness