The Collaborative International Dictionary of English v.0.48:

Excitability \Ex*cit"a*bil"i*ty\, n. [Cf. F. excitabilit['e].]
   1. The quality of being readily excited; proneness to be
      affected by exciting causes.
      [1913 Webster]

   2. (Physiol.) The property manifested by living organisms,
      and the elements and tissues of which they are
      constituted, of responding to the action of stimulants;
      irritability; as, nervous excitability.
      [1913 Webster]

WordNet (r) 3.0 (2006):

excitability
    n 1: excessive sensitivity of an organ or body part [syn:
         excitability, irritability]
    2: being easily excited [syn: excitability, excitableness,
       volatility]