While it's not a specific "thing", gravity is the force that pulls objects towards each other. It's the reason water flows downhill, rivers empty into the ocean, and rain falls from the sky.

Here are some other terms related to water movement:

* Downhill flow: This describes the general direction of water movement due to gravity.

* Runoff: This refers to water flowing over the land surface, often after rain.

* Discharge: This is the volume of water flowing past a point in a river or stream.

* Current: This is the movement of water in a larger body like an ocean or lake.

Let me know if you'd like to know more about any of these terms!