The view that a single God exists is known as:

Theism is the belief in the existence of one or more gods, and specifically, monotheism is the belief in a single God. In this context, theism refers to the belief in a personal God who interacts with the world.

Here's a breakdown of the other terms:

Deism: Belief in the existence of a creator who does not intervene in the universe or the affairs of humans.
Atheism: The absence of belief in the existence of any gods or deities.
So, theism covers the belief in a single God, as in monotheism, or multiple gods, as in polytheism.