The Collaborative International Dictionary of English v.0.48:

Grail \Grail\, n. [OF. graal, greal, greet, F. graal, gr['e]al,
   LL. gradalis, gradale, prob. derived fr. L. crater bowl,
   mixing vessel, Gr. krath`r. See Crater.]
   A broad, open dish; a chalice; -- only used of the Holy
   Grail.
   [1913 Webster]

   Note: The Holy Grail, according to some legends of the Middle
         Ages, was the cup used by our Savior in dispensing the
         wine at the last supper; and according to others, the
         platter on which the paschal lamb was served at the
         last Passover observed by our Lord. This cup, according
         to the legend, if appoached by any but a perfectly pure
         and holy person, would be borne away and vanish from
         the sight. The quest of the Holy Grail was to be
         undertaken only by a knight who was perfectly chaste in
         thought, word, and act.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Holy \Ho"ly\, a. [Compar. Holier; superl. Holiest.] [OE.
   holi, hali, AS. h[=a]lig, fr. h[ae]l health, salvation,
   happiness, fr. h[=a]l whole, well; akin to OS. h?lag, D. & G.
   heilig, OHG. heilac, Dan. hellig, Sw. helig, Icel. heilagr.
   See Whole, and cf. Halibut, Halidom, Hallow,
   Hollyhock.]
   1. Set apart to the service or worship of God; hallowed;
      sacred; reserved from profane or common use; holy vessels;
      a holy priesthood. "Holy rites and solemn feasts."
      --Milton.
      [1913 Webster]

   2. Spiritually whole or sound; of unimpaired innocence and
      virtue; free from sinful affections; pure in heart; godly;
      pious; irreproachable; guiltless; acceptable to God.
      [1913 Webster]

            Now through her round of holy thought
            The Church our annual steps has brought. --Keble.
      [1913 Webster]

   Holy Alliance (Hist.), a league ostensibly for conserving
      religion, justice, and peace in Europe, but really for
      repressing popular tendencies toward constitutional
      government, entered into by Alexander I. of Russia,
      Francis I. of Austria, and Frederic William III. of
      Prussia, at Paris, on the 26th of September, 1815, and
      subsequently joined by all the sovereigns of Europe,
      except the pope and the king of England.

   Holy bark. See Cascara sagrada.

   Holy Communion. See Eucharist.

   Holy family (Art), a picture in which the infant Christ,
      his parents, and others of his family are represented.

   Holy Father, a title of the pope.

   Holy Ghost (Theol.), the third person of the Trinity; the
      Comforter; the Paraclete.

   Holy Grail. See Grail.

   Holy grass (Bot.), a sweet-scented grass (Hierochloa
      borealis and Hierochloa alpina). In the north of Europe
      it was formerly strewed before church doors on saints'
      days; whence the name. It is common in the northern and
      western parts of the United States. Called also vanilla
      grass or Seneca grass.

   Holy Innocents' day, Childermas day.

   Holy Land, Palestine, the birthplace of Christianity.

   Holy office, the Inquisition.

   Holy of holies (Script.), the innermost apartment of the
      Jewish tabernacle or temple, where the ark was kept, and
      where no person entered, except the high priest once a
      year.

   Holy One.
      (a) The Supreme Being; -- so called by way of emphasis. "
          The Holy One of Israel." --Is. xliii. 14.
      (b) One separated to the service of God.

   Holy orders. See Order.

   Holy rood, the cross or crucifix, particularly one placed,
      in churches. over the entrance to the chancel.

   Holy rope, a plant, the hemp agrimony.

   Holy Saturday (Eccl.), the Saturday immediately preceding
      the festival of Easter; the vigil of Easter.

   Holy Spirit, same as Holy Ghost (above).

   Holy Spirit plant. See Dove plant.

   Holy thistle (Bot.), the blessed thistle. See under
      Thistle.

   Holy Thursday. (Eccl.)
      (a) (Episcopal Ch.) Ascension day.
      (b) (R. C. Ch.) The Thursday in Holy Week; Maundy
          Thursday.

   Holy war, a crusade; an expedition carried on by Christians
      against the Saracens in the Holy Land, in the eleventh,
      twelfth, and thirteenth centuries, for the possession of
      the holy places.

   Holy water (Gr. & R. C. Churches), water which has been
      blessed by the priest for sacred purposes.

   Holy-water stoup, the stone stoup or font placed near the
      entrance of a church, as a receptacle for holy water.

   Holy Week (Eccl.), the week before Easter, in which the
      passion of our Savior is commemorated.

   Holy writ, the sacred Scriptures. " Word of holy writ."
      --Wordsworth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Holy Grail
    n 1: (legend) chalice used by Christ at the Last Supper [syn:
         grail, Holy Grail, Sangraal]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "Holy Grail":
   Agnus Dei, Host, Pieta, Sanctus bell, Sangraal, ark, asperger,
   asperges, aspergillum, bambino, beadroll, beads, candle, censer,
   chaplet, ciborium, cross, crucifix, cruet, eucharistial,
   holy cross, holy water, holy-water sprinkler, icon, incensory,
   matzo, menorah, mezuzah, mikvah, monstrance, osculatory,
   ostensorium, paschal candle, pax, phylacteries, prayer shawl,
   prayer wheel, pyx, relics, rood, rosary, sacramental,
   sacred relics, sacring bell, shofar, sukkah, tabernacle, tallith,
   thurible, urceole, veronica, vigil light, votive candle