(chemistry) any theory in which all matter is composed of tiny discrete finite indivisible indestructible particles; "the ancient Greek philosophers Democritus and Epicurus held atomic theories of the universe"
(psychology) a theory that reduces all mental phenomena to simple elements (sensations and feelings) that form complex ideas by association
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"