The Collaborative International Dictionary of English v.0.48:

Cheerfulness \Cheer"ful*ness\, n.
   Good spirits; a state of moderate joy or gayety; alacrity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "cheerfulness":
   Leibnizian optimism, Pollyannaism, auspiciousness, beatification,
   beatitude, beneficence, benevolence, benignancy, benignity,
   bewitchment, blessedness, bliss, blissfulness, bright outlook,
   bright side, brightness, cheer, cheeriness, cloud nine,
   delectation, delight, ecstasy, ecstatics, elation, enchantment,
   exaltation, exhilaration, exuberance, fair weather, favorableness,
   felicity, fortunateness, gaiety, gladness, glee, good auspices,
   good omen, happiness, heaven, high spirits, intoxication, joy,
   joyance, joyfulness, luckiness, millennialism, optimism,
   optimisticalness, overhappiness, overjoyfulness, paradise,
   perfectibilism, perfectionism, philosophical optimism,
   propitiousness, prosperousness, rapture, ravishment, rosy outlook,
   seventh heaven, silver lining, sunniness, sunny side, sunshine,
   transport, unalloyed happiness, utopianism