The Collaborative International Dictionary of English v.0.48:

Fugacity \Fu*gac"i*ty\, a. [L fugacitas: cf. F. fugacit['e].]
   1. The quality of being fugacious; fugaclousness; volatility;
      as, fugacity of spirits. --Boyle.
      [1913 Webster]

   2. Uncertainty; instability. --Johnson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fugacity
    n 1: the tendency of a gas to expand or escape
    2: the lack of enduring qualities (used chiefly of plant parts)
       [syn: fugacity, fugaciousness]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "fugacity":
   caducity, changeableness, corruptibility, death, ephemerality,
   ephemeralness, evanescence, finitude, fleetingness, impermanence,
   impermanency, instability, momentariness, mortality, mutability,
   perishability, transience, transiency, transientness,
   transitoriness, volatility