The definition of theism is simple in itself, Theism is the belief in a deity. Beyond this there are many different forms of Theism. Atheism is the opposite if Theism.
There are many forms of Theism, and cannot be confined to one specific religion or world view. Christianity, Judaism and Islam are mono-theistic religions, believing in one deity. Religions that believe in multiple gods are considered polytheistic. All these religions fall under the umbrella of Theism, though this is not to say that the belief systems of these religions are the same.
Dualism is commonly associated with Theism through our time period because it embraces the notion of a soul. Both materialists and dualists could be Theists. There was, and is, no universal rule defining which world view a person subscribes to. What made a person a Theist was their belief in a deity, not their beliefs in materialism or dualism.