The bias direction of a piece of woven fabric, usually referred to simply as “the bias”, is at 45 degrees to its warp and weft threads. Every piece of woven fabric has two biases, perpendicular to each other. Non-woven fabrics such as felt or interfacing do not have a bias.
Bias tape or bias binding is a narrow strip of fabric, cut on the bias. The strip’s fibers, being at 45 degrees to the length of the strip, makes it stretchier as well as more fluid and more drapeable compared to a strip that is cut on the grain.