Twill is a type of textile weave with a pattern of diagonal parallel ribs (in contrast with a satin and plain weave). This is done by passing the weft thread over one or more warp threads and then under two or more warp threads and so on, with a "step" or offset between rows to create the characteristic diagonal pattern. Because of this structure, twills generally drape well. Examples of twill fabric are chino, drill, denim, gabardine, tweed and serge.

In a twill weave, each weft or filling yarn floats across the warp yarns in a progression of interlacings to the right or left, forming a distinct diagonal line. This diagonal line is also known as a wale. A float is the portion of a yarn that crosses over two or more yarns from the opposite direction.

A twill weave requires three or more harnesses, depending on its complexity. A twill weave is the second most basic weave that can be made on a fairly simple loom.

Twill weave is often designated as a fraction—such as 2/1—in which the numerator indicates the number of harnesses that are raised (and, thus, threads crossed), in this example, two, and the denominator indicates the number of harnesses that are lowered when a filling yarn is inserted, in this example one. The fraction 2/1 would be read as "two up, one down." The minimum number of harnesses needed to produce a twill can be determined by totaling the numbers in the fraction. For the example described, the number of harnesses is three. (The fraction for plain weave is 1/1.)