What is a tributary of river called?

A tributary of a river is a smaller stream or river that flows into a larger river. The larger river is called the "main stem" or "parent river". Tributaries can be thought of as the branches of a tree, with the main stem being the trunk.

The point where a tributary meets the main stem is called the "confluence". The confluence of two rivers can be a major geographic feature, and can be a site of human settlement and economic activity.