A boutonnière is a floral decoration worn by men, typically a single flower or bud. The word comes from the French word for buttonhole, which is the British term.
While worn frequently in the past, boutonnières are now usually reserved for special occasions for which formal wear is standard, such as at proms, homecomings, funerals, and weddings. (Women who wear jackets on these occasions also often may wear "buttonholes", but more typically a woman would wear a corsage.) Nowadays, a lapel pin is worn more often than flowers on business suits.