It is an area of land. All water that falls on that land flows into one river. It can flow directly into the river or go through tributaries (smaller rivers that flow into the bigger river) first.
This term can have two main meanings:
- Meaning drainage divide, the line that separates neighbouring drainage basins
- Meaning drainage basin in North American usage. (an area of land where surface water converges)
There are also a number of figurative meanings as a metaphor.