Source : Webster's Revised Unabridged Dictionary (1913)

Cheerfulness \Cheer"ful*ness\, n.
   Good spirits; a state of moderate joy or gayety; alacrity.

Source : WordNet®

cheerfulness
     n 1: the quality of being cheerful and dispelling gloom; "flowers
          added a note of cheerfulness to the drab room" [syn: {cheer}]
          [ant: {uncheerfulness}]
     2: a feeling of spontaneous good spirits; "his cheerfulness
        made everyone feel better" [syn: {blitheness}] [ant: {cheerlessness}]