Trousers that are made of denim are called jeans:
- You shouldn’t wear jeans for a job interview.
- I like wearing jeans and a T-shirt on weekdays.
Denim is the name of the fabric:
- I’ve bought a denim bag.
- Do you like denim shirts?
Jeans are always used in the plural form (just like trousers or pants), so the word jean is incorrect.
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