The Collaborative International Dictionary of English v.0.48:

Mobility \Mo*bil"i*ty\ (m[-o]*b[i^]l"[i^]*t[y^]), n. [L.
   mobilitas: cf. F. mobilit['e].]
   1. The quality or state of being mobile; as, the mobility of
      a liquid, of an army, of the populace, of features, of a
      muscle. --Sir T. Browne.
      [1913 Webster]

   2. The mob; the lower classes. [Humorous] --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mobility
    n 1: the quality of moving freely [ant: immobility]