From 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]
From 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]