What does the terminology Theism mean?

Answer 1

In the broadest sense, it is the belief that at least one god exists.

In a more technical sense, it's used to describe a particular sort of monotheism (one that is largely compatible with, say, any of the Abrahamic religions): the belief that not only does God exist but that God takes an active part in the universe. It is to be distinguished from Deism, which holds that it's possible to rationally determine that God exists (by observing the natural universe) without relying upon "revelations" or "faith". Deism also usually incorporates the belief that God created the universe and hasn't monkeyed with it since.

If the universe were a building, a good analogy would be that the Deist version of God was the architect and contractor. The Theist version has him as that and also the superintendent.

Answer 2

Theism:

It is a belief in a God as creator and supreme ruler of the universe, who is above his creation but present and active in the governance and organization of the world and the universe.

Answer 3

Theism is the belief in one deity, (at least), such as monotheism - the monotheistic doctrine concerning the nature of conceiving one god being present, governing the universe.