Let’s start with a simple definition:
A watershed is the area of land that catches rain and snow and drains or seeps into a marsh, stream, river, lake or groundwater.
Here’s a more formal definition:
“The watershed is defined as a unit of natural or disturbed land on which all the water that falls (or emanates from springs) collects by gravity and fails to evaporate and runs off via a common outlet. The watershed is the basic unit of water supply.”
–Peter E. Black, Watershed Hydrology, Second Edition, 1996.
Here’s a more holistic view:
“That area of land, a bounded hydrologic system, within which all living things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community.”
–John Wesley Powell, explorer and scientist