From Longman Dictionary of Contemporary Englishtop hatˌtop ˈhat noun [countable] 
DCCa man’s tall black or grey hat, now worn only on formal occasions
DCCa man’s tall black or grey hat, now worn only on formal occasionsExamples from the Corpus
top hat• I used to wear dinner suits with cufflinks and a top hat.• Beneath a top hat his face is pasty and bloated.• At most weddings the bride wears a veil but at this wedding the bride was wearing a beautiful top hat.• Then the wind rose again and plucked his top hat off his head and sent it bowling among the stones.• Gentlemen will wear morning dress with top hat, or service dress. 