das Kopfweh
headache
Pain in the head area; another, somewhat more colloquial term for headaches.
Ich habe schreckliches Kopfweh und kann mich nicht konzentrieren.
I have a terrible headache and can't concentrate.

headache
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headache
Pain in the head area; another, somewhat more colloquial term for headaches.
Ich habe schreckliches Kopfweh und kann mich nicht konzentrieren.
I have a terrible headache and can't concentrate.

The word is a simple compound of 'der Kopf' (head) and 'das Weh' (pain/ache). 'Das Weh' is an older word for pain that still occurs today in words like 'Heimweh' (homesickness) or 'Zahnweh' (toothache).
Nach dem langen Flug hatte sie starkes Kopfweh.
After the long flight, she had a bad headache.
Similar to 'Hunger' (hunger) or 'Durst' (thirst), 'Kopfweh' (headache) is often used without an article, especially when saying that you 'have' it.
Ich kann heute nicht mitkommen, ich habe leider Kopfweh.
I can't come along today; unfortunately, I have a headache.
'Kopfweh' (headache) is a common word in everyday life and spoken language. It often sounds a bit more personal or less medically technical.
Meine Oma klagt oft über Kopfweh, wenn das Wetter sich ändert.
My grandma often complains of headaches when the weather changes.
You 'hat' Kopfweh (have a headache), 'bekommt' Kopfweh (get a headache), or 'klagt über' Kopfweh (complain of a headache). To describe the pain, you can use adjectives like 'stark' (strong/severe), 'leicht' (mild), or 'schrecklich' (terrible).
Wenn ich zu wenig trinke, bekomme ich schnell Kopfweh.
If I don't drink enough, I quickly get a headache.
headache
'Kopfschmerz' and 'Kopfweh' both mean pain in the head. 'Kopfschmerz' sounds somewhat more medical or neutral; it is very often used in the plural as 'Kopfschmerzen'. 'Kopfweh' is a bit more colloquial and often sounds simpler in everyday speech. In normal conversations, both are easy to understand. At the doctor's or in a more formal text, 'Kopfschmerzen' is usually more natural than 'Kopfweh'.

Nach dem lauten Konzert hatte Lisa Kopfweh und wollte nach Hause.
After the loud concert, Lisa had a headache and wanted to go home.
Beim Arzt beschreibt er seine Kopfschmerzen genau.
At the doctor's, he describes his headaches in detail.