The Collaborative International Dictionary of English v.0.48:

Agony \Ag"o*ny\, n.; pl. Agonies. [L. agonia, Gr. ?, orig. a
   contest, fr. ?: cf. F. agonie. See Agon.]
   1. Violent contest or striving.
      [1913 Webster]

            The world is convulsed by the agonies of great
            nations.                              --Macaulay.
      [1913 Webster]

   2. Pain so extreme as to cause writhing or contortions of the
      body, similar to those made in the athletic contests in
      Greece; and hence, extreme pain of mind or body; anguish;
      paroxysm of grief; specifically, the sufferings of Christ
      in the garden of Gethsemane.
      [1913 Webster]

            Being in an agony he prayed more earnestly. --Luke
                                                  xxii. 44.
      [1913 Webster]

   3. Paroxysm of joy; keen emotion.
      [1913 Webster]

            With cries and agonies of wild delight. --Pope.
      [1913 Webster]

   4. The last struggle of life; death struggle.
      [1913 Webster]

   Syn: Anguish; torment; throe; distress; pangs; suffering.

   Usage: Agony, Anguish, Pang. These words agree in
          expressing extreme pain of body or mind. Agony denotes
          acute and permanent pain, usually of the whole
          system., and often producing contortions. Anguish
          denotes severe pressure, and, considered as bodily
          suffering, is more commonly local (as anguish of a
          wound), thus differing from agony. A pang is a
          paroxysm of excruciating pain. It is severe and
          transient. The agonies or pangs of remorse; the
          anguish of a wounded conscience. "Oh, sharp convulsive
          pangs of agonizing pride!" --Dryden.
          [1913 Webster]

WordNet (r) 3.0 (2006):

agony
    n 1: intense feelings of suffering; acute mental or physical
         pain; "an agony of doubt"; "the torments of the damned"
         [syn: agony, torment, torture]
    2: a state of acute pain [syn: agony, suffering,
       excruciation]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "agony":
   aching heart, affliction, agonizingness, agony of mind, anguish,
   atrocious pain, bale, bitterness, bleeding heart, broken heart,
   care, carking care, crucifixion, crushing, death agonies,
   death groan, death rattle, death struggle, death throes, deathbed,
   deathwatch, depression, depth of misery, desolateness, desolation,
   despair, distress, dolor, dying breath, excruciatingness,
   excruciation, extremity, final extremity, grief, heartache,
   heartbreak, heartfelt grief, heartgrief, heartsickness,
   heavy heart, infelicity, lamentation, languishment, last agony,
   last breath, last gasp, martyrdom, martyrization, melancholia,
   melancholy, misery, moribundity, pain, pangs, passion, pining,
   prostration, rack, sadness, sorrow, sorrowing, suffering,
   suicidal despair, throes, throes of death, torment, tormentingness,
   torture, torturousness, trouble, woe, wretchedness



Easton's 1897 Bible Dictionary:

Agony
   contest; wrestling; severe struggling with pain and suffering.
   Anguish is the reflection on evil that is already past, while
   agony is a struggle with evil at the time present. It is only
   used in the New Testament by Luke (22:44) to describe our Lord's
   fearful struggle in Gethsemane.

     The verb from which the noun "agony" is derived is used to
   denote an earnest endeavour or striving, as "Strive [agonize] to
   enter" (Luke 13:24); "Then would my servants fight" [agonize]
   (John 18:36). Comp. 1 Cor. 9:25; Col. 1:29; 4:12; 1 Tim. 6:12; 2
   Tim. 4:7, where the words "striveth," "labour," "conflict,"
   "fight," are the renderings of the same Greek verb.