Containing no lead. In practice you will meet this word in one place: petrol (American English: gasoline). Unleaded petrol is the ordinary fuel every car uses now, and the word exists only because of history — for most of the twentieth century a lead compound was added to petrol to help engines run smoothly, and it turned out to be poisoning people, especially children. Countries banned it one by one, so through the 1980s and 1990s pumps were labelled 'unleaded' to tell drivers this was the safe new kind.

Today leaded petrol is gone almost everywhere, so 'unleaded' has quietly become just a name rather than a contrast. British drivers still say 'fill it up with unleaded' and Americans still see 'unleaded' on the pump, even though there is nothing else to choose. You can use it as a noun on its own — 'twenty pounds of unleaded, please' — which is very common at the petrol station. In Taiwan, note that we usually just say 無鉛汽油 or name the octane number (95, 98), so students may never have needed the English word until they rent a car abroad.

It is also used, much more rarely, of other things that once contained lead: unleaded paint, unleaded crystal glass. And there is a small joke you may hear in American cafés: 'unleaded' for decaf coffee — coffee with the powerful stuff taken out. It is playful and old-fashioned, but people do say it.

Pronunciation is worth a moment: the 'lead' here is the metal, so it rhymes with 'red', not with 'read/reed'. Say un-LED-id.