Longman Dictionary of Contemporary English
crash helmet
noun
EXAMPLES FROM CORPUS
▪ Never seen him without his crash helmet on; could be an albino or a Rastafarian for all he knew.
▪ Tom had never before ridden pillion on a motorbike, but Andy the neighbour had a spare crash helmet.
Wiktionary
crash helmet
n. A helmet worn by motorcyclists worn to protect the head in case of an accident.
WordNet
crash helmet
n. a padded helmet worn by people riding bicycles or motorcycles; protects the head in case of accidents