life jacket
noun [ C ] uk/ˈlaɪf ˌdʒæk.ɪt/ us/ˈlaɪf ˌdʒæk.ɪt/
救生衣，救生背心
a piece of equipment, like a jacket without sleeves, that is filled with air or light material and is designed to help you float if you fall into water
See also: life preserver