noun
Refers to the very top or crown of the head. Often used in contexts like striking the top of the head or describing a bald spot.
転んで脳天を強く打った。
I fell and hit the top of my head hard.
彼は脳天がはげている。
He is bald on the crown of his head.
頭頂 is a more formal or anatomical term for the top of the head, while 脳天 is more colloquial and often used in everyday situations.
Compound of 脳 (brain) and 天 (top/heaven), literally 'brain top'. The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.