Pants:
(1) [AmE] a piece of lothing that covers you from your waist to your feet and has a separate part for each leg. 長褲子, ie. trousers [BrE]
(2) [BrE] A piece of underwear that covers the area between your waist and the top of your legs 短襯褲、内褲
根據bbc.com.uk
Trousers versus Pants
In America, a "pair of pants" refers to an item of clothing used to cover the entire lower torso and pelvic region of the body, but often also includes clothing that covers from the hip right down to the ankle. If the article of clothing doesn't cover the knees and below, it is usually referred to as "shorts" or something, even though it's really still a pair of pants.
In England, a "pair of pants" refers to an item of clothing used to cover the main areas of the lower region of the body (from hip down to just below the groin). Pants are usually covered (at least in public) with another article of clothing called "trousers". Trousers cover the body from below the torso to above the feet, and Americans call trousers pants.
What Americans refer to as "shorts" or "underwear" is actually a pair of pants to the British. Americans are much less specific in the definition than the British. In America, trousers are called pants and pants are called shorts or underpants. In Great Britain, underpants are called pants and pants are called trousers. British speaking people generally find this more amusing than American speaking people, who oftentimes don't understand why the Brits are laughing at them.