A dark area where light from a light source is blocked by an opaque object is called:

• An opaque object prevents light from passing through it. 
• As a result, when an opaque object is placed in front of a light source, it blocks the light coming from the source and causes a dark region having the shape of an object to appear on the surface below the opaque object. 
• This dark region is basically called the object's shadow.

Therefore, we can say that shadow is a dark area where light from a light source is blocked by an opaque object.