Bias

The word “bias” refers to fabric bias: 45° from the grainline. Cutting a piece of fabric this way (“bias cut”) means aligning the grainline arrow of the pattern piece to a line placed 45° from the fabric grainline. Bias cut woven pieces have more elasticity that can be useful in a garment (for example for the upper part of a collar).
You can easily find bias-cut ribbon (bias binding) at the haberdashery. Most of the time it is pre-folded and really useful to finish armholes or necklines neatly.