The noun 头盔 (tóukuī) means 'helmet.' It combines 头 (head) with 盔 (helmet, armor for the head). It refers to any protective headgear, whether used in military, sports, construction, or motorcycle riding contexts. It is also used when describing the shape of objects that resemble helmets.
骑摩托车一定要戴头盔。
qí mótuōchē yídìng yào dài tóukuī.
You must wear a helmet when riding a motorcycle.
工地上所有工人都必须佩戴安全头盔。
gōngdì shàng suǒyǒu gōngrén dōu bìxū pèidài ānquán tóukuī.
All workers on the construction site must wear safety helmets.
他买了一个新头盔,准备周末去骑自行车。
tā mǎi le yí gè xīn tóukuī, zhǔnbèi zhōumò qù qí zìxíngchē.
He bought a new helmet, planning to go cycling this weekend.