From Longman Dictionary of Contemporary Englishhard hatˌhard ˈhat noun [countable] 
TBCa protective hat, worn especially by workers in places where buildings are being built
TBCa protective hat, worn especially by workers in places where buildings are being builtExamples from the Corpus
hard hat• If hydrogen sulfide or some other poisonous gas is detected, Donahue dons an airtight breathing device and a hard hat.• A hard hat can protect you from having a metal spike go through your skull and into your brain.• I had to put my hard hat on and go to work.• When no vapors are present, he simply wears the hard hat.From Longman Business Dictionaryhard hatˈhard hat noun [countable]1a protective hat worn by buildersHard hats are now required by the Occupational Safety and Health Administration for all construction workers.2American English informalJOB a worker in the building industryPeople working in television tend not to feel at home with hard hats and blue collars. 