The Collaborative International Dictionary of English v.0.48:

Trinity \Trin"i*ty\, n. [OE. trinitee, F. trinit['e], L.
   trinitas, fr. trini three each. See Trinal.]
   1. (Christian Theol.) The union of three persons (the Father,
      the Son, and the Holy Ghost) in one Godhead, so that all
      the three are one God as to substance, but three persons
      as to individuality.
      [1913 Webster]

   2. Any union of three in one; three units treated as one; a
      triad, as the Hindu trinity, or Trimurti.
      [1913 Webster]

   3. Any symbol of the Trinity employed in Christian art,
      especially the triangle.
      [1913 Webster]

   Trinity House, an institution in London for promoting
      commerce and navigation, by licensing pilots, ordering and
      erecting beacons, and the like.

   Trinity Sunday, the Sunday next after Whitsunday; -- so
      called from the feast held on that day in honor of the
      Holy Trinity.

   Trinity term. (Law) See the Note under Term, n., 5.
      [1913 Webster]

WordNet (r) 3.0 (2006):

trinity
    n 1: the cardinal number that is the sum of one and one and one
         [syn: three, 3, III, trio, threesome, tierce,
         leash, troika, triad, trine, trinity, ternary,
         ternion, triplet, tercet, terzetto, trey, deuce-
         ace]
    2: the union of the Father and Son and Holy Ghost in one Godhead
       [syn: Trinity, Holy Trinity, Blessed Trinity, Sacred
       Trinity]
    3: three people considered as a unit [syn: trio, threesome,
       triad, trinity]

The Devil's Dictionary (1881-1906):

TRINITY, n.  In the multiplex theism of certain Christian churches,
three entirely distinct deities consistent with only one.  Subordinate
deities of the polytheistic faith, such as devils and angels, are not
dowered with the power of combination, and must urge individually
their claims to adoration and propitiation.  The Trinity is one of the
most sublime mysteries of our holy religion.  In rejecting it because
it is incomprehensible, Unitarians betray their inadequate sense of
theological fundamentals.  In religion we believe only what we do not
understand, except in the instance of an intelligible doctrine that
contradicts an incomprehensible one.  In that case we believe the
former as a part of the latter.