An area of land whose boundaries are determined by the direction of water flow is called a watershed or drainage basin.

Here's a breakdown:

* Watershed: A watershed is a geographical area where all the surface water drains to a common outlet, such as a river, lake, or ocean. It's like a giant bathtub, with the edges of the tub representing the boundaries of the watershed and the drain representing the outlet.

* Drainage Basin: This term is often used interchangeably with watershed. It emphasizes the collection and flow of water within the defined area.

The boundaries of a watershed are determined by the highest points in the surrounding land, which are called divides or watersheds. Water flowing from one side of a divide will drain into a different watershed than water flowing from the other side.