The bias is located at a 45 degree angle to the straight grain and the cross grain of a woven fabric.
About the Bias
Technically, any diagonal cut that does not lie on the straight grain or the cross grain of a woven fabric is a bias cut. However, in conventional usage when people talk about a bias cut they are actually referring to the true bias. It is the true bias that intersects the straight grain and the cross grain at a 45 degree angle. Every piece of woven fabric has two true biases which are perpendicular to each other.
Garments cut on the bias tend to have different properties to those cut on the straight or cross grain, for example those cut on the bias will have more stretch and appear more fluid. These properties can be advantageous as the fit will be more forgiving, however, the instability of the fabric will make it more difficult to work with.