A brassiere is a bra: the piece of underwear that supports a woman’s chest. The long word came into English from French. Almost everyone now says bra; brassiere sounds older and more formal, the kind of word you still see on a shop label or in a catalogue. If a student only learns bra, they will be fine in speech. Brassiere is simply the full name of the same thing.

胸罩（口語幾乎都說 bra；brassiere 是較完整、較正式的說法。中國大陸較常說「文胸」）