Source : Webster's Revised Unabridged Dictionary (1913)

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.

            He is always serious, yet there is about his manner
            a graceful ease.                      --Macaulay.

   2. Really intending what is said; being in earnest; not
      jesting or deceiving. --Beaconsfield.

   3. Important; weighty; not trifling; grave.

            The holy Scriptures bring to our ears the most
            serious things in the world.          --Young.

   4. Hence, giving rise to apprehension; attended with danger;
      as, a serious injury.

   Syn: Grave; solemn; earnest; sedate; important; weighty. See
        {Grave}. -- {Se"ri*ous*ly}, adv. -- {Se"ri*ous*ness}, n.

Source : WordNet®

seriousness
     n 1: an earnest and sincere feeling [syn: {earnestness}, {sincerity}]
     2: the quality of arousing fear or distress; "he learned the
        seriousness of his illness" [syn: {distressfulness}]
     3: the trait of being serious; "a lack of solemnity is not
        necessarily a lack of seriousness"- Robert Rice [syn: {earnestness},
         {serious-mindedness}, {sincerity}] [ant: {frivolity}]