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]

Moby Thesaurus II by Grady Ward, 1.0:

116 Moby Thesaurus words for "interjection":
   Parthian shot, address, adjectival, adjective, adverb, adverbial,
   adversative conjunction, affirmation, allegation, answer,
   apostrophe, aside, assertion, attributive, averment, comment,
   conjunction, conjunctive adverb, coordinating conjunction,
   copulative, copulative conjunction, correlative conjunction, crack,
   cry, declaration, dictum, disjunctive, disjunctive conjunction,
   ejaculation, embedment, encroachment, entrance, entrenchment,
   episode, exclamation, exclamatory noun, expression, form class,
   form word, function class, gerundive, graft, grafting, greeting,
   impaction, impactment, impingement, implantation, imposition,
   incursion, infiltration, infix, infixion, influx, infringement,
   infusion, injection, inoculation, inroad, insert, insertion,
   insinuation, intercalation, interference, interlineation,
   interlocution, interloping, interpolation, interposition,
   interposure, interruption, intervention, introduction,
   intromission, intrusion, invasion, irruption, mention, note,
   obiter dictum, observation, obtrusion, parenthesis, part of speech,
   participle, particle, past participle, penetration,
   perfect participle, perfusion, phrase, position, preposition,
   present participle, pronouncement, question, reflection, remark,
   say, saying, sentence, side remark, statement, subjoinder,
   subordinating conjunction, tessellation, thought, tossing-in,
   transplant, transplantation, trespass, trespassing, unlawful entry,
   utterance, verbal adjective, word