The Collaborative International Dictionary of English v.0.48:

Altar \Al"tar\, n. [OE. alter, auter, autier, fr. L. altare, pl.
   altaria, altar, prob. fr. altus high: cf. OF. alter, autier,
   F. autel. Cf. Altitude.]
   1. A raised structure (as a square or oblong erection of
      stone or wood) on which sacrifices are offered or incense
      burned to a deity.
      [1913 Webster]

            Noah builded an altar unto the Lord.  --Gen. viii.
                                                  20.
      [1913 Webster]

   2. In the Christian church, a construction of stone, wood, or
      other material for the celebration of the Holy Eucharist;
      the communion table.
      [1913 Webster]

   Note: Altar is much used adjectively, or as the first part of
         a compound; as, altar bread or altar-bread.
         [1913 Webster]

   Altar cloth or

   Altar-cloth, the cover for an altar in a Christian church,
      usually richly embroidered.

   Altar cushion, a cushion laid upon the altar in a Christian
      church to support the service book.

   Altar frontal. See Frontal.

   Altar rail, the railing in front of the altar or communion
      table.

   Altar screen, a wall or partition built behind an altar to
      protect it from approach in the rear.

   Altar tomb, a tomb resembling an altar in shape, etc.

   Family altar, place of family devotions.

   To lead (as a bride) to the altar, to marry; -- said of a
      woman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

altar
    n 1: the table in Christian churches where communion is given
         [syn: altar, communion table, Lord's table]
    2: a raised structure on which gifts or sacrifices to a god are
       made

The Devil's Dictionary (1881-1906):

ALTAR, n.  The place whereupon the priest formerly raveled out the
small intestine of the sacrificial victim for purposes of divination
and cooked its flesh for the gods.  The word is now seldom used,
except with reference to the sacrifice of their liberty and peace by a
male and a female tool.

    They stood before the altar and supplied
    The fire themselves in which their fat was fried.
    In vain the sacrifice! -- no god will claim
    An offering burnt with an unholy flame.
                                                           M.P. Nopput