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

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "altar":
   Communion table, altar carpet, altar desk, altar facing,
   altar of prothesis, altar rail, altar side, altar slab,
   altar stair, altar stone, altarpiece, ancona, bomos, chancel table,
   credence, eschara, frontal, gradin, hestia, holy table, mensal,
   missal stand, predella, prothesis, retable, retablo, rood altar,
   scrobis, superaltar



Easton's 1897 Bible Dictionary:

Altar
   (Heb. mizbe'ah, from a word meaning "to slay"), any structure of
   earth (Ex. 20:24) or unwrought stone (20:25) on which sacrifices
   were offered. Altars were generally erected in conspicuous
   places (Gen. 22:9; Ezek. 6:3; 2 Kings 23:12; 16:4; 23:8; Acts
   14:13). The word is used in Heb. 13:10 for the sacrifice offered
   upon it--the sacrifice Christ offered.

     Paul found among the many altars erected in Athens one bearing
   the inscription, "To the unknown God" (Acts 17:23), or rather
   "to an [i.e., some] unknown God." The reason for this
   inscription cannot now be accurately determined. It afforded the
   apostle the occasion of proclaiming the gospel to the "men of
   Athens."

     The first altar we read of is that erected by Noah (Gen.
   8:20). Altars were erected by Abraham (Gen. 12:7; 13:4; 22:9),
   by Isaac (Gen. 26:25), by Jacob (33:20; 35:1, 3), and by Moses
   (Ex. 17:15, "Jehovah-nissi").

     In the tabernacle, and afterwards in the temple, two altars
   were erected.

     (1.) The altar of burnt offering (Ex. 30:28), called also the
   "brasen altar" (Ex. 39:39) and "the table of the Lord" (Mal.
   1:7).

     This altar, as erected in the tabernacle, is described in Ex.
   27:1-8. It was a hollow square, 5 cubits in length and in
   breadth, and 3 cubits in height. It was made of shittim wood,
   and was overlaid with plates of brass. Its corners were
   ornamented with "horns" (Ex. 29:12; Lev. 4:18).

     In Ex. 27:3 the various utensils appertaining to the altar are
   enumerated. They were made of brass. (Comp. 1 Sam. 2:13, 14;
   Lev. 16:12; Num. 16:6, 7.)

     In Solomon's temple the altar was of larger dimensions (2 Chr.
   4:1. Comp. 1 Kings 8:22, 64; 9:25), and was made wholly of
   brass, covering a structure of stone or earth. This altar was
   renewed by Asa (2 Chr. 15:8). It was removed by Ahaz (2 Kings
   16:14), and "cleansed" by Hezekiah, in the latter part of whose
   reign it was rebuilt. It was finally broken up and carried away
   by the Babylonians (Jer. 52:17).

     After the return from captivity it was re-erected (Ezra 3:3,
   6) on the same place where it had formerly stood. (Comp. 1 Macc.
   4:47.) When Antiochus Epiphanes pillaged Jerusalem the altar of
   burnt offering was taken away.

     Again the altar was erected by Herod, and remained in its
   place till the destruction of Jerusalem by the Romans (70 A.D.).

     The fire on the altar was not permitted to go out (Lev. 6:9).

     In the Mosque of Omar, immediately underneath the great dome,
   which occupies the site of the old temple, there is a rough
   projection of the natural rock, of about 60 feet in its extreme
   length, and 50 in its greatest breadth, and in its highest part
   about 4 feet above the general pavement. This rock seems to have
   been left intact when Solomon's temple was built. It was in all
   probability the site of the altar of burnt offering. Underneath
   this rock is a cave, which may probably have been the granary of
   Araunah's threshing-floor (1 Chr. 21:22).

     (2.) The altar of incense (Ex. 30:1-10), called also "the
   golden altar" (39:38; Num. 4:11), stood in the holy place
   "before the vail that is by the ark of the testimony." On this
   altar sweet spices were continually burned with fire taken from
   the brazen altar. The morning and the evening services were
   commenced by the high priest offering incense on this altar. The
   burning of the incense was a type of prayer (Ps. 141:2; Rev.
   5:8; 8:3, 4).

     This altar was a small movable table, made of acacia wood
   overlaid with gold (Ex. 37:25, 26). It was 1 cubit in length and
   breadth, and 2 cubits in height.

     In Solomon's temple the altar was similar in size, but was
   made of cedar-wood (1 Kings 6:20; 7:48) overlaid with gold. In
   Ezek. 41:22 it is called "the altar of wood." (Comp. Ex.
   30:1-6.)

     In the temple built after the Exile the altar was restored.
   Antiochus Epiphanes took it away, but it was afterwards restored
   by Judas Maccabaeus (1 Macc. 1:23; 4:49). Among the trophies
   carried away by Titus on the destruction of Jerusalem the altar
   of incense is not found, nor is any mention made of it in Heb.
   9. It was at this altar Zacharias ministered when an angel
   appeared to him (Luke 1:11). It is the only altar which appears
   in the heavenly temple (Isa. 6:6; Rev. 8:3,4).


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