WordWeb Online

1. Being full of or promoting cheer; having or showing good spirits
   "her cheerful nature"; "a cheerful greeting"; "a cheerful room"; "as cheerful as anyone confined to a hospital bed could be"
    
2. Pleasantly (even unrealistically) optimistic
   - pollyannaish, upbeat, Pollyannaish

See also: beaming, beamish, blithe, blithesome [archaic], buoyant, cheer, cheerfulness, cheery, chipper, chirpy [informal], debonair, debonaire, gay, glad, happy, jaunty, lighthearted, light-hearted, lightsome, optimistic, perky, playful, smiley, smiling, sunniness, sunny, sunshine, sunshiny, twinkly, unserious

Antonym: uncheerful

Encyclopedia: Cheerful