The Collaborative International Dictionary of English v.0.48:

Calm \Calm\ (k[aum]m), a. [Compar. Calmer (-[~e]r); superl.
   Calmest (-[e^]st)]
   1. Not stormy; without motion, as of winds or waves; still;
      quiet; serene; undisturbed. "Calm was the day." --Spenser.
      [1913 Webster]

            Now all is calm, and fresh, and still. --Bryant.
      [1913 Webster]

   2. Undisturbed by passion or emotion; not agitated or
      excited; tranquil; quiet in act or speech. "Calm and
      sinless peace." --Milton. "With calm attention." --Pope.
      [1913 Webster]

            Such calm old age as conscience pure
            And self-commanding hearts ensure.    --Keble.

   Syn: Still; quiet; undisturbed; tranquil; peaceful; serene;
        composed; unruffled; sedate; collected; placid.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Calm \Calm\ (k[aum]m), n. [OE. calme, F. calme, fr. It. or Sp.
   calma (cf. Pg. calma heat), prob. fr. LL. cauma heat, fr. Gr.
   kay^ma burning heat, fr. kai`ein to burn; either because
   during a great heat there is generally also a calm, or
   because the hot time of the day obliges us seek for shade and
   quiet; cf. Caustic]
   Freedom from motion, agitation, or disturbance; a cessation
   or absence of that which causes motion or disturbance, as of
   winds or waves; tranquility; stillness; quiet; serenity.
   [1913 Webster]

         The wind ceased, and there was a great calm. --Mark.
                                                  iv. 39.
   [1913 Webster]

         A calm before a storm is commonly a peace of a man's
         own making.                              --South.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Calm \Calm\, v. i. [imp. & p. p. Calmed (k[aum]md); p. pr. &
   vb. n. Calming.] [Cf. F. calmer. See Calm, n.]
   1. To make calm; to render still or quiet, as elements; as,
      to calm the winds.
      [1913 Webster]

            To calm the tempest raised by Eolus.  --Dryden.
      [1913 Webster]

   2. To deliver from agitation or excitement; to still or
      soothe, as the mind or passions.
      [1913 Webster]

            Passions which seem somewhat calmed.  --Atterbury.

   Syn: To still; quiet; appease; allay; pacify; tranquilize;
        soothe; compose; assuage; check; restrain.
        [1913 Webster]

WordNet (r) 3.0 (2006):

calm
    adj 1: not agitated; without losing self-possession; "spoke in a
           calm voice"; "remained calm throughout the uproar"; "he
           remained serene in the midst of turbulence"; "a serene
           expression on her face"; "she became more tranquil";
           "tranquil life in the country" [syn: calm,
           unagitated, serene, tranquil]
    2: (of weather) free from storm or wind; "calm seas" [ant:
       stormy]
    n 1: steadiness of mind under stress; "he accepted their
         problems with composure and she with equanimity" [syn:
         composure, calm, calmness, equanimity] [ant:
         discomposure]
    2: wind moving at less than 1 knot; 0 on the Beaufort scale
       [syn: calm air, calm]
    v 1: make calm or still; "quiet the dragons of worry and fear"
         [syn: calm, calm down, quiet, tranquilize,
         tranquillize, tranquillise, quieten, lull, still]
         [ant: agitate, charge, charge up, commove,
         excite, rouse, turn on]
    2: make steady; "steady yourself" [syn: steady, calm,
       becalm]
    3: become quiet or calm, especially after a state of agitation;
       "After the fight both men need to cool off."; "It took a
       while after the baby was born for things to settle down
       again." [syn: calm, calm down, cool off, chill out,
       simmer down, settle down, cool it]
    4: cause to be calm or quiet as by administering a sedative to;
       "The patient must be sedated before the operation" [syn:
       sedate, calm, tranquilize, tranquillize,
       tranquillise] [ant: arouse, brace, energise,
       energize, perk up, stimulate]

Moby Thesaurus II by Grady Ward, 1.0:

272 Moby Thesaurus words for "calm":
   abnegation, abstinence, accordance, allay, alleviate, anticyclone,
   appease, assuage, at peace, at rest, aweless, balm, balmy, becalm,
   blank, blankminded, bloodless, calm down, calmness, cease,
   cloistered, collected, compose, composed, composure, conciliate,
   concordant, conservatism, consistency, consonance, constancy,
   constraint, continence, continuity, control, cool, cool off,
   coolheaded, coolness, correspondence, cradle, dead calm,
   deathlike calm, defuse, die down, dispassion, dispassionate,
   doldrums, dulcify, dwindle, dwindling, ease, easy, easygoing, ebb,
   ebbing, empty, empty-headed, equability, equable, equanimity,
   equilibrium, even, even out, even-tempered, even-tenored, evenness,
   expected, expecting, fatuous, firm, flat calm, gentle, gentleness,
   golden mean, halcyon, halt, happy medium, homogeneity,
   horse latitudes, hush, hushed, idyllic, impartiality, impassive,
   imperturbable, inactive, inane, incogitant, inexcitable, isolated,
   judiciousness, juste-milieu, lay, lay the dust, lenity,
   levelheaded, lull, make one easy, meden agan, middle way, mild,
   mildness, mitigate, moderateness, moderation, moderationism,
   molder, moldering, mollify, monolithism, motionless, nerveless,
   neutrality, nirvanic, nonchalant, nonviolence, nothing in excess,
   oblivious, oily calm, orderly, pacific, pacifism, pacify, passive,
   pastoral, peace, peaceable, peaceful, peacefulness, peacetime,
   persistence, philosophical, phlegmatic, piping, placate, placid,
   placidity, placidness, poised, possessed, pour balm into,
   pour balm on, propitiate, prudence, quell, quiesce, quiescent,
   quiet, quieten, quietistic, relax, relaxed, relieve, repose,
   reposeful, reposing, rest, restful, resting, restraint, rock,
   rock to sleep, rock-steady, sang-froid, secluded, sedate,
   self-abnegation, self-control, self-controlled, self-denial,
   self-possessed, self-possession, self-restraint, sequestered,
   sequestrated, serene, serenity, settle, sheltered, smooth,
   smooth down, smooth over, smoothen, sobriety, soft, soothe,
   soother, stability, stabilize, stable, staid, staunch,
   steadfastness, steadiness, steady, steady-handed, steady-nerved,
   steel-nerved, still, still as death, stillish, stillness, stilly,
   stoic, stoical, stolid, stop, strong-nerved, subdue, subside,
   subsiding, temperance, temperateness, thoughtfree, thoughtless,
   together, tranquil, tranquilize, tranquillity, unagitated,
   unamazed, unastonished, unastounded, unawed, unbewildered,
   unblenching, unblinking, undazed, undazzled, undisturbed,
   undumbfounded, unexcessiveness, unextravagance, unextremeness,
   unfaltering, unflappable, unflinching, unideaed, uniformity,
   unimpressed, unintellectual, unity, unmarveling, unmoved,
   unnervous, unoccupied, unperturbed, unquivering, unreasoning,
   unruffled, unruffledness, unshaken, unshaky, unshrinking,
   unstirring, unstrained, unsurprised, unthinking, untremulous,
   untroubled, unwavering, unwondering, vacant, vacuous, via media,
   wane, waning, windless, windlessness, without a tremor,
   wonderless