Trousers is the general term, standard in British English for any such garment. Slacks refers to casual or semi-formal trousers specifically — not jeans, not suit trousers — and now sounds noticeably dated. American English generally uses pants as the everyday word, which in British English means underwear.
The core difference
One is the category; the other is a style within it, and an ageing word.
- trousers — the general British term for the garment.
- slacks — casual or semi-formal trousers; dated in current use.
- pants — the everyday American word; underwear in British English.
The transatlantic trap
This is one of the reliable British-American misunderstandings. Telling a British audience you spilled something on your pants describes a more private accident than intended. British English uses trousers for the outer garment and pants or underwear for what goes beneath.
Why trousers is plural
Like scissors, glasses and tongs, it names a two-part object and takes a plural verb. You need a pair of trousers — "a trouser" only appears in the clothing trade, as in trouser leg.
Frequently asked questions
Do British people say pants for trousers?
No. In British English pants means underwear. The outer garment is trousers.
Are slacks still used?
The word is understood but sounds dated. It refers to casual or semi-formal trousers rather than jeans or suit trousers.
Why is trousers plural?
Like scissors and glasses, it names a two-part object, so it takes a plural verb and a pair of.