a part of an engine that consists of a short cylinder that fits inside a tube and moves up and down or backwards and forwards to make other parts of the engine move 活塞 
Oxford Collocations DictionaryPiston is used before these nouns:- engine
Word Originearly 18th cent.: from French, from Italian pistone, variant of pestone ‘large pestle’, augmentative of pestello ‘pestle’.
