From Longman Dictionary of Contemporary Englishldoce_201_alife jacketˈlife ˌjacket noun [countable] British English 
TTWa piece of clothing that can be filled with air and worn around your upper body to stop you from sinking in the water SYN life vest American English
TTWa piece of clothing that can be filled with air and worn around your upper body to stop you from sinking in the water SYN life vest American EnglishExamples from the Corpus
life jacket• School was very important, a life jacket in chaos.• Despite my life jacket, I was pinned under water by the down-surging river hydraulic. 