Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

Source : WordNet®

interchangeably
     adv : in an interchangeable manner; "these terms can be used
           interchangeably"