Both are British English, but they mean different things: "pants" are a kind of underwear worn underneath "trousers". There is some local variation, though: around Manchester, for example, you may hear people saying "pants" instead of "trousers".

Not quite. Underwear is underwear here in the US and pants are also trousers. But trousers is used less as a synonym. Trousers has an old-fashioned ring to it for us Americans. 'Underpants' is a baby talk.

Trousers.

Cool，thank u. I suppose that pants is a British expression which is equally the same as underwear to American : )

I am not sure about British English, but in Canada we use both. I would say that pants is more commonly used. Also, men wear trousers and women would wear pants. Men and women can both wear pants but women never wear trousers.