A term used to define both the diagonal structural design of textile weave as well as the cloth created from that weave process. Twill materials generally have a firm and strong form and are used in suits, work clothing, linings and pockets. There are various types of twill including gabardine, cheviot and serge.
Twill is a specific type of weave used to create fabric. It is woven by passing the weft thread over and under several warp threads rather than just one, as with plain weave. In addition, each row is offset from the previous one, creating a diagonal ribbing pattern in the material (this can easily be seen on the reverse side of denim). Twill weave has a front and a back, with the front side having a more distinct rib and being more attractive in general. It creates a more pliable material with better drapability, wrinkle resistance, and durability than plain-woven fabric. Denim, a twill-woven fabric, is woven with either a right hand twill, left hand twill, or broken twill.
Here is a picture of twill: