A more thorough explanation:

Definition: The ancient-lights doctrine is a common-law principle that allows a landowner to acquire an easement preventing a neighbor from building an obstruction that blocks light from passing through the landowner's window after 20 years of uninterrupted use. The window or other opening is called an ancient light. This doctrine is rarely applied in the United States.

Example: If a landowner has received sunlight across adjoining property for a specified period of time, the landowner is entitled to continue to receive unobstructed access to sunlight across the property. This is known as the ancient-lights doctrine.

Explanation: The ancient-lights doctrine allows a landowner to protect their right to a reasonable amount of light, free from interruption, over adjoining land. If a landowner has received sunlight across adjoining property for a specified period of time, they can claim an easement preventing a neighbor from building an obstruction that blocks light from passing through the landowner's window. This doctrine is rarely applied in the United States, but it is still used in some countries like England.