The Collaborative International Dictionary of English v.0.48:

Interjection \In`ter*jec"tion\, n. [L. interjectio: cf. F.
   interjection. See Interject.]
   [1913 Webster]
   1. The act of interjecting or throwing between; also, that
      which is interjected.
      [1913 Webster]

            The interjection of laughing.         --Bacon.
      [1913 Webster]

   2. (Gram.) A word or form of speech thrown in to express
      emotion or feeling, as O! Alas! Ha ha! Begone! etc.
      Compare Exclamation.
      [1913 Webster]

            An interjection implies a meaning which it would
            require a whole grammatical sentence to expound, and
            it may be regarded as the rudiment of such a
            sentence. But it is a confusion of thought to rank
            it among the parts of speech.         --Earle.
      [1913 Webster]

            How now! interjections? Why, then, some be of
            laughing, as, ah, ha, he!             --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interjection
    n 1: an abrupt emphatic exclamation expressing emotion [syn:
         ejaculation, interjection]
    2: the action of interjecting or interposing an action or remark
       that interrupts [syn: interjection, interposition,
       interpolation, interpellation]