Source : Webster's Revised Unabridged Dictionary (1913)

Fixity \Fix"i*ty\ (-[i^]*t[y^]), n. [Cf. F. fixit['e].]
   1. Fixedness; as, fixity of tenure; also, that which is
      fixed.

   2. Coherence of parts. --Sir I. Newton.

Source : WordNet®

fixity
     n 1: the quality of being fixed in place [syn: {fastness}, {fixedness},
           {fixture}, {secureness}]
     2: the quality of being incapable of mutation; "Darwin
        challenged the fixity of species" [syn: {immutability}, {immutableness}]
        [ant: {mutability}, {mutability}]