IPA: /ˈklɪrˌhɛdəd.nəs/
KK: /ˈklɪrˌhɛdɪdnəs/
The state of being able to think clearly and make good decisions.
After a good night's sleep, she felt a sense of clearheadedness that helped her solve the problem easily.
Clearheadedness → It is formed from "clear" (from Old English "clǣre", meaning free from darkness or obscurity) and "headedness" (from "head" meaning the part of the body where the mind is located, combined with the suffix "-ness" meaning the state or quality of). The word "clearheadedness" refers to the state of having a clear and rational mind.
Think of being 'free from darkness' ('clear') in your 'mind' ('head') — that's what clearheadedness means.