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®

serious
     adj 1: concerned with work or important matters rather than play or
            trivialities; "a serious student of history"; "a
            serious attempt to learn to ski"; "gave me a serious
            look"; "a serious young man"; "are you serious or
            joking?"; "Don't be so serious!" [ant: {frivolous}]
     2: of great consequence; "marriage is a serious matter"
     3: causing fear or anxiety by threatening great harm; "a
        dangerous operation"; "a grave situation"; "a grave
        illness"; "grievous bodily harm"; "a serious wound"; "a
        serious turn of events"; "a severe case of pneumonia"; "a
        life-threatening disease" [syn: {dangerous}, {grave}, {grievous},
         {severe}, {life-threatening}]
     4: appealing to the mind; "good music"; "a serious book" [syn:
        {good}]
     5: completely lacking in playfulness [syn: {unplayful}, {sober}]
        [ant: {playful}]
     6: requiring effort or concentration; complex and not easy to
        answer or solve; "raised serious objections to the
        proposal"; "the plan has a serious flaw"