IPA: /ˈtʃɪə.ri.nəs/
KK: /ˈtʃɪr.i.nəs/
The quality or state of being happy and cheerful.
Her cheeriness brightened everyone's day at the office.
Cheeriness is formed from "cheer" (from Old English *cēar*, meaning joy or gladness) and the suffix "-iness" (meaning the state or quality of). The word "cheeriness" refers to the state of being cheerful or full of joy.
Think of the word 'cheer' which means joy, and remember that 'cheeriness' is the state of being joyful.