The Collaborative International Dictionary of English v.0.48:

Solemnity \So*lem"ni*ty\, n.; pl. Solemnities. [L. solemnitas,
   solennitas: cf. F. solennit['e], solemnit['e], OF. also
   sollempnit['e].]
   1. A rite or ceremony performed with religious reverence;
      religious or ritual ceremony; as, the solemnity of a
      funeral, a sacrament.
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            Great was the cause; our old solemnities
            From no blind zeal or fond tradition rise,
            But saved from death, our Argives yearly pay
            These grateful honors to the god of day. --Pope.
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   2. ceremony adapted to impress with awe.
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            The forms and solemnities of the last judgment.
                                                  --Atterburry.
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   3. Ceremoniousness; impressiveness; seriousness; grave
      earnestness; formal dignity; gravity.
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            With much glory and great solemnity.  --Chaucer.
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            The statelines and gravity of the Spaniards shows
            itself in the solemnity of their language.
                                                  --Addison.
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            These promises were often made with great solemnity
            and confirmed with an oath.           --J. Edwards.
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   4. Hence, affected gravity or seriousness.
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            Solemnity 's a cover for a sot.       --Young.
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   5. Solemn state or feeling; awe or reverence; also, that
      which produces such a feeling; as, the solemnity of an
      audience; the solemnity of Westminster Abbey.
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   6. (Law) A solemn or formal observance; proceeding according
      to due form; the formality which is necessary to render a
      thing done valid.
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WordNet (r) 3.0 (2006):

solemnity
    n 1: a trait of dignified seriousness [syn: sedateness,
         staidness, solemnity, solemness]
    2: a solemn and dignified feeling [syn: gravity, solemnity]
       [ant: levity]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "solemnity":
   aridity, augustness, baccalaureate service, barrenness, bleakness,
   bloodlessness, buckram, celebration, ceremonial, ceremoniousness,
   ceremony, characterlessness, circumstance, colorlessness,
   commencement, consequence, convocation, courtliness, darkness,
   deadness, dignifiedness, dignity, dismalness, dragginess,
   dreariness, dryness, dullness, dustiness, duty, earnestness,
   effeteness, elevation, emptiness, empty formality, etiolation,
   exercise, exercises, extrinsicality, flatness, form,
   form of worship, formal, formality, formalization, formula,
   formulary, function, gloom, gloominess, graduation,
   graduation exercises, grandeur, gravity, grimness, heaviness,
   heraldry, hollowness, holy rite, impersonality, importance,
   impressiveness, inanity, inaugural, inauguration, inexcitability,
   initiation, insipidity, insipidness, institution, jejunity,
   kingliness, leadenness, lifelessness, liturgy, loftiness,
   lordliness, lowness of spirit, majesty, mode of worship,
   moderation, momentousness, mummery, mystery, no joke,
   no laughing matter, nobility, observance, office, order of worship,
   ordinance, paleness, pallor, performance, pointlessness, pokiness,
   pomp, pomposity, ponderousness, practice, prescribed form, pride,
   pride of bearing, pride of place, primness, princeliness,
   proud bearing, regality, religious ceremony, reserve, rigidness,
   rite, rite de passage, rite of passage, ritual, ritual observance,
   rituality, sacrament, sacramental, sedateness, seriousness,
   service, slowness, sober-mindedness, soberness, sobersidedness,
   sobersides, sobriety, solemnization, solemnness, somberness,
   spiritlessness, staidness, starchiness, state, stateliness,
   sterility, stiffness, stiltedness, stodginess, stuffiness,
   stylization, sublimity, superficiality, taciturnity, tastelessness,
   tediousness, temperance, uninterestingness, unliveliness, vapidity,
   vapidness, venerability, wearifulness, wearisomeness, weight,
   weightiness, woodenness, worthiness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SOLEMNITY. The formality established by law to render a contract, agreement,
or other act valid.
     2. A marriage, for example, would not be valid if made in jest, and
without solemnity. Vide Marriage, and Dig. 4, 1, 7; Id. 45, 1, 30.