IPA: /ˈhɛdeɪk/
KK: /ˈhɛdˌeɪk/
A pain or discomfort in the head that can vary in intensity and duration.
After a long day at work, I often get a headache.
Headache → It is formed from "head" (from Old English "heafod", meaning the upper part of the body) and "ache" (from Old English "æce", meaning pain). The word "headache" refers to pain located in the head.
Think of the 'upper part of the body' ('head') experiencing 'pain' ('ache') — that's what a headache means.