From Longman Dictionary of Contemporary Englishhelmethel‧met /ˈhelmɪt/ ●●● W3 noun [countable] 
DCCTTa strong hard hat that soldiers, motorcycle riders, the police etc wear to protect their heads → crash helmet, pith helmet
DCCTTa strong hard hat that soldiers, motorcycle riders, the police etc wear to protect their heads → crash helmet, pith helmetExamples from the Corpus
helmet• I remember having a scorpion on my helmet once.• He had no helmet, no overcoat, no weapon and no boots.• There was a sentry in a steel helmet at the door.• First I thought that a bullet had hit me on the helmet and somehow bounced off.• With his other hand he took the helmet off.• The helmet is designed to limit the rotational forces affecting the head during an impact and is currently undergoing bench tests.• In the event of a threat to security, they would grab their helmets and weapons and rush to the trouble spot.Origin helmet (1400-1500) Old French helme “helmet” 