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 Sunday
    n 1: eighth Sunday after Easter