snap fastener
(also `press stud, Brit infml 英式口语作 popper), device made of two small round metal or plastic parts that are pressed together to fasten dresses, skirts, etc 子母扣儿; 摁扣儿
the press stud on the collar of his evening shirt 他晚礼服衬衫领口上的摁扣儿
the poppers on a child´s pyjamas 儿童睡衣上的子母扣儿.