Twill is a kind of cloth. What makes it twill is not what it is made of, but the way the threads are crossed when it is woven. In plain cloth, the threads go over one, under one, over one — very even, and the surface looks flat. In twill, each thread goes over two or three and then under one, and each new row starts one step to the side. Because every row is shifted, the crossing points line up into slanting lines that run across the cloth, like many small diagonal steps. If you look closely at a pair of jeans, you can see those little slanted lines — denim is a twill. So is gabardine, so is the thick cotton in many work trousers and coats.

The word is mostly used by people who work with cloth: shop staff, tailors, factory people, sewing hobbyists. A label may say "cotton twill" or "wool twill", meaning the material and then the weave. Ordinary speakers rarely say "twill" in daily talk; they just say "jeans" or "a jacket". Twill cloth is usually strong and it hides dirt and creases better than plain weave, which is why it is used for hard-wearing clothes. There is also a verb "to twill" — to weave cloth this way — but you will almost never meet it outside a textile book.

One more thing, though: because twill has a right side with clear diagonal lines and a back side that looks different, cloth people also talk about which way the lines lean. It is a small detail, but it is the whole reason the word exists.