Denim is a kind of heavy cotton material that's used to make jeans. You might think you look really cool in your rhinestone-studded denim jacket . Go ahead and keep thinking that.
Denim is usually dark blue, and mostly used to make durable work wear, like jeans and overalls. In some places, people call blue jeans denims. The word comes from a French phrase serge de Nîmes, "serge from Nîmes, France." "Serge" is a sturdy twill fabric, and Nîmes is a town where it was famously made. Denim was first used in American English around 1850.