[syn: seriousness, earnestness, serious-mindedness, sincerity]
The Collaborative International Dictionary of English v.0.48:
Serious \Se"ri*ous\, a. [L. serius: cf. F. s['e]rieux, LL.
   seriosus.]
   1. Grave in manner or disposition; earnest; thoughtful;
      solemn; not light, gay, or volatile.
      [1913 Webster]
            He is always serious, yet there is about his manner
            a graceful ease.                      --Macaulay.
      [1913 Webster]
   2. Really intending what is said; being in earnest; not
      jesting or deceiving. --Beaconsfield.
      [1913 Webster]
   3. Important; weighty; not trifling; grave.
      [1913 Webster]
            The holy Scriptures bring to our ears the most
            serious things in the world.          --Young.
      [1913 Webster]
   4. Hence, giving rise to apprehension; attended with danger;
      as, a serious injury.
      [1913 Webster]
   Syn: Grave; solemn; earnest; sedate; important; weighty. See
        Grave.
        [1913 Webster] -- Se"ri*ous*ly, adv. --
        Se"ri*ous*ness, n.
        [1913 Webster]
WordNet (r) 3.0 (2006):
seriousness
    n 1: an earnest and sincere feeling [syn: earnestness,
         seriousness, sincerity]
    2: the quality of arousing fear or distress; "he learned the
       seriousness of his illness" [syn: seriousness,
       distressfulness]
    3: the trait of being serious; "a lack of solemnity is not
       necessarily a lack of seriousness"- Robert Rice [syn:
       seriousness, earnestness, serious-mindedness,
       sincerity] [ant: frivolity, frivolousness]
Moby Thesaurus II by Grady Ward, 1.0:
77 Moby Thesaurus words for "seriousness":
   abandon, ardency, ardor, commitment, committedness, decidedness,
   decision, decisiveness, decorousness, dedication, definiteness,
   demureness, determinateness, determination, determinedness,
   devotedness, devotion, devoutness, earnest, earnestness, faith,
   faithfulness, fervency, fervidness, fervor, fidelity, fire,
   formality, gravity, grimness, heartiness, heat, heatedness,
   impassionedness, intensity, intentness, long face, loyalty,
   moderation, no joke, no laughing matter, obstinacy, passion,
   passionateness, perseverance, persistence, purpose, purposefulness,
   relentlessness, resoluteness, resolution, resolve, resolvedness,
   sedateness, self-will, sincerity, single-mindedness,
   sober-mindedness, soberness, sobersidedness, sobersides, sobriety,
   solemnity, solemnness, somberness, spirit, staidness,
   straight face, temperance, tenacity, thoughtfulness,
   total commitment, vehemence, warmth, weightiness, will, zeal