helmet       
noun [ C ] uk/ˈhel.mət/ us/ˈhel.mət/
B2 头盔,安全帽
a strong, hard hat that covers and protects the head
  a crash helmet 防撞头盔 
 a bicycle helmet 摩托车头盔 
 - More examples
 - It's very risky to ride your bike without wearing a cycle helmet.
 - To keep the helmet in position, fasten the strap beneath the chin.
 - Is the strap on this helmet adjustable?
 - He pulled on his helmet and straddled the motorbike.
 - You cannot come onto the building site unless you're wearing a helmet.