The Collaborative International Dictionary of English v.0.48:

Tit \Tit\, n.
   1. A small horse. --Tusser.
      [1913 Webster]

   2. A woman; -- used in contempt. --Burton.
      [1913 Webster]

   3. A morsel; a bit. --Halliwell.
      [1913 Webster]

   4. [OE.; cf. Icel. titter a tit or small bird. The word
      probably meant originally, something small, and is perhaps
      the same as teat. Cf. Titmouse, Tittle.] (Zool.)
      (a) Any one of numerous species of small singing birds
          belonging to the families Paridae and
          Leiotrichidae; a titmouse.
      (b) The European meadow pipit; a titlark.
          [1913 Webster]

   Ground tit. (Zool.) See Wren tit, under Wren.

   Hill tit (Zool.), any one of numerous species of Asiatic
      singing birds belonging to Siva, Milna, and allied
      genera.

   Tit babbler (Zool.), any one of several species of small
      East Indian and Asiatic timaline birds of the genus
      Trichastoma.

   Tit for tat. [Probably for tip for tap. See Tip a slight
      blow.] An equivalent; retaliation.

   Tit thrush (Zool.), any one of numerous species of Asiatic
      and East Indian birds belonging to Suthora and allied
      genera. In some respects they are intermediate between the
      thrushes and titmice.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tit for tat
    n 1: an equivalent given in return

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "tit for tat":
   agency, alternation, balance, ballast, battledore and shuttlecock,
   blow for blow, change, coaction, commutation,
   complementary distribution, concurrence, consideration,
   cooperation, counterbalance, counterchange, counterpoise,
   counterweight, cross fire, delegation, deputation, deputyship,
   displacement, engagement, equipoise, equivalent, exchange,
   give-and-take, interaction, interchange, intercommunication,
   intercourse, interlacing, intermeshing, intermutation, interplay,
   intertwining, interweaving, interworking, law of retaliation,
   lex talionis, like for like, makeweight, measure for measure, mesh,
   meshing, mutual admiration, mutual support, mutual transfer,
   mutuality, offset, permutation, power of attorney, quid pro quo,
   reciprocality, reciprocation, reciprocity, replacement,
   representation, retaliation, seesaw, setoff,
   something for something, something in return, something of value,
   subrogation, substitution, supersedence, superseding, supersedure,
   supersession, supplantation, supplanting, supplantment, switch,
   talion, transposal, transposition, vicariousness