Acorn nut
HARDWARE NUT
Cap nut; Dome nut
An acorn nut, also referred to as crown hex nut, blind nut, cap nut, domed cap nut, or dome nut (UK), is a nut that has a domed end on one side. When used together with a threaded fastener with an external male thread, the domed end encloses the external thread, either to protect the thread or to protect nearby objects from contact with the thread.