The Collaborative International Dictionary of English v.0.48:

Interchangeable \In`ter*change"a*ble\, a. [Cf. OF.
   entrechangeable.]
   [1913 Webster]
   1. Admitting of exchange or mutual substitution.
      "Interchangeable warrants." --Bacon.
      [1913 Webster]

   2. Following each other in alternate succession; as, the four
      interchangeable seasons. --Holder. --
      In`ter*change"a*ble*ness, n. -- In`ter*change"a*bly,
      adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interchangeableness
    n 1: the quality of being capable of exchange or interchange
         [syn: exchangeability, interchangeability,
         interchangeableness, fungibility] [ant:
         unexchangeability]