Source : Webster's Revised Unabridged Dictionary (1913)

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

            The interjection of laughing.         --Bacon.

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

            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.

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

Source : WordNet®

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