The Collaborative International Dictionary of English v.0.48:

Spontaneity \Spon`ta*ne"i*ty\ (sp[o^]n`t[.a]*n[=e]"[i^]*t[y^]),
   n.; pl. Spontaneities (-t[i^]z). [Cf. F.
   spontan['e]it['e].]
   1. The quality or state of being spontaneous, or acting from
      native feeling, proneness, or temperament, without
      constraint or external force.
      [1913 Webster]

            Romney Leigh, who lives by diagrams,
            And crosses not the spontaneities
            Of all his individual, personal life
            With formal universals.               --Mrs.
                                                  Browning.
      [1913 Webster]

   2. (Biol.)
      (a) The tendency to undergo change, characteristic of both
          animal and vegetable organisms, and not restrained or
          checked by the environment.
      (b) The tendency to activity of muscular tissue, including
          the voluntary muscles, when in a state of healthful
          vigor and refreshment.
          [1913 Webster]

WordNet (r) 3.0 (2006):

spontaneity
    n 1: the quality of being spontaneous and coming from natural
         feelings without constraint; "the spontaneity of his
         laughter" [syn: spontaneity, spontaneousness]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "spontaneity":
   abandon, autonomousness, autonomy, ease, extemporaneousness,
   free will, gratuitousness, indeliberation, independence,
   involuntariness, naturalness, offhandedness, self-action,
   self-activity, self-determination, snap decision, snap judgment,
   snapshot, spontaneousness, uncalculatedness, undeliberateness,
   unforcedness, unpremeditation, unrestraint, voluntariness,
   voluntarism, voluntaryism, volunteer, volunteering