Brightness of disposition and outward good humor.
Brightness of disposition and outward good humor — a manner that lifts one's own and others' spirits.
Differs in kind from the rest: it names a disposition or outward manner, not a felt inner emotion. Where happiness, joy, and gladness describe what you feel, cheerfulness describes how you seem and behave — bright, good-humored, spirit-lifting — and it is trait-like and durable (one can be cheerful by nature, even through hardship). Outward-directed: it raises others' spirits. Milder and steadier than the boisterous merriment of mirth or glee.
From cheerful (c. 1400, “full of good spirits”) + -ness. Cheer (c. 1200) first meant “the face, countenance,” from Late Latin cara “face”; it shifted to “mood” and then to the positive “gladness, good spirits” by c. 1400.
The shift lives in the root cheer: “face” → “mood” (good or bad) → predominantly “good spirits.” No reliable recent-generation shift is sourced.