The Collaborative International Dictionary of English v.0.48:

nonrigid \nonrigid\ adj. (Aeronautics)
   Not rigid; especially, designating an airship having a shape
   maintained only by internal gas pressure and without a
   supporting structure. Opposite of rigid.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

nonrigid
    adj 1: designating an airship having a shape maintained only by
           internal gas pressure and without a supporting structure
           [ant: rigid]