A more thorough explanation:

Definition: Ad medium filum aquae is a legal term that means "to the thread of the water" or "to the central line or middle of a stream." It refers to the ownership reach of a riparian proprietor.

Example: If a person owns property that borders a river, they have the right to use the water up to the middle of the stream. This means that they can fish, swim, or boat in the water up to the point where the middle of the stream is located.

Explanation: The example illustrates how ad medium filum aquae is used to determine the extent of a riparian proprietor's ownership rights. By using the central line or middle of the stream as a boundary, it is clear how far the owner's rights extend into the water.