The Collaborative International Dictionary of English v.0.48:

Serene \Se*rene"\, a. [L. serenus to grow dry, Gr. ??? hot,
   scorching.]
   1. Bright; clear; unabscured; as, a serene sky.
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            The moon serene in glory mounts the sky. --Pope.
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            Full many a gem of purest ray serene
            The dark unfathomed caves of ocean bear. --Gray.
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   2. Calm; placid; undisturbed; unruffled; as, a serene aspect;
      a serene soul. --Milton.
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   Note: In several countries of Europe, Serene is given as a
         tittle to princes and the members of their families;
         as, His Serene Highness.
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   Drop serene. (Med.) See Amaurosis. --Milton.
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The Collaborative International Dictionary of English v.0.48:

Serene \Se*rene"\, n.
   1. Serenity; clearness; calmness. [Poetic.] "The serene of
      heaven." --Southey.
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            To their master is denied
            To share their sweet serene.          --Young.
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   2. [F. serein evening dew or damp. See Serein.] Evening
      air; night chill. [Obs.] "Some serene blast me." --B.
      Jonson.
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The Collaborative International Dictionary of English v.0.48:

Serene \Se*rene"\, v. t. [L. serenare.]
   To make serene.
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         Heaven and earth, as if contending, vie
         To raise his being, and serene his soul. --Thomson.
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WordNet (r) 3.0 (2006):

serene
    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: completely clear and fine; "serene skies and a bright blue
       sea"

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "serene":
   at peace, bloodless, bucolic, calm, clear, cloudless, collected,
   composed, concordant, cool, coolheaded, diaphanous, dispassionate,
   easy, easygoing, equable, even, even-tempered, halcyon, idyllic,
   imperturbable, levelheaded, light, lightish, lightsome, lucent,
   lucid, luculent, noiseless, nonchalant, orderly, pacific, pastoral,
   peaceable, peaceful, peacetime, pellucid, philosophical, piping,
   placid, poised, quiescent, quiet, relucent, reposeful, restful,
   resting, self-possessed, soft, still, temperate, tranquil,
   translucent, translucid, transparent, unclouded, undisturbed,
   unexcitable, unflappable, unobscured, unperturbed, unruffled,
   untroubled