The name "denim" is an anglicized short form of "Serge de Nîmes" (fabric from Nîmes, a town in France, where this kind of fabric was produced).
This fabric from Nîmes was introduced into the US by Levi Strauss who used it to produce solid trousers which later became known as jeans.
Denim is a woven fabric of the twill type in which the weft thread passes under two or more warp threads with an offset of one step between adjacent rows so that a diagonal ribbing of the fabric is achieved. The fabric is generally made with 100% cotton. Some sorts of denim, however, also comprise an elastic component such as spandex. This type of denim is called stretch denim.
Typically, the warp threads are dyed with an indigo dye in the distinctive blue color of "blue jeans", while the the weft threads are left white. In the garment produced from this fabric, the dyed warp-side faces outwards and shows the color whereas the inside shows the white weft threads.
The weft threads, however, are not thourougly dyed, but left white in their core. By abrasion during wearing and washing, the white core appears under the dyed surface so that the color of the fabric fades. This gives used garments made from denim, especially jeans, a unique look. Such a used look, however, may also be achieved by different washings during the production, from a simple stonewash process up to an extensive treatment by which special effects are achieved.
Levi's not only offer denim jeans, but also other clothing made from or including denim.
Levi's also have developed a weatherproof version of denim for their Commuter collection.