valve: [14] The etymological notion underlying valve is of a door opening and closing. The word was adapted from Latin valva, which denoted one of the sections of a folding or revolving door, and may have had links with volvere ‘roll’ (source of English revolve). It carried its original meaning with it into English, but it was not used at all widely until various metaphorical senses, such as ‘flap controlling the flow of a fluid’ and ‘half of a shell’, evolved. The electronic valve is so called because current can flow in only one direction through it; the usage dates from the early 20th century. [valve etymology, valve origin, 英语词源]