The term theism simply refers to a person's beliefs about god. Various prefixes are sometimes added to this term to clarify more specifically people's beliefs about their god(s).
a + theism [atheism] = belief in no god
mono + theism [monotheism] = belief in only one god
poly + theism [polytheism] = belief in more than one god.
On this web page, we are using the term "theism" in a more narrow sense of monotheism--belief in only one god. There are three major world religions which are monotheistic: Judaism, Christianity, and Islam. In addition, two other world views are monotheistic: deism and theistic existentialism. These are explained through the following links:
Theistic Media Reviews