Q: What is a cast shadow?

A: A cast shadow is a shadow that is cast by a shape onto other surfaces. It originates on the part of a form that is hidden from the light and is projected onto a plane in accordance with the laws of perspective. The cast shadow is not always an exact likeness of the form casting a shadow because of variables of the light source, and the relationship of the form to the direction of the light source, as well as the relationship to the plane on which the shadow falls.

Comments

I don't get it: what else are shadows other than "cast" shadows.

Is there a difference between a shadow that is beneth or aside an object vs a shadow that falls on a wall, for instance?

Cast shadows are areas of objects which are not receiving light from some light source because some object is in the way. That object is receiving the light and causing the "shadow". The "shadow" can take on interesting shapes depending on the shape of the "receiving" surface.