Theism is the belief that God, or a god, or gods, really exist. That is the whole of it — it is a word for the position itself, not for any particular religion. A Christian, a Muslim, a Jew and a Hindu are all theists, even though what they believe about that god differs enormously.

The word belongs to philosophy and to serious religious discussion, not to daily conversation. Ordinary people say 'I believe in God'; philosophers say 'theism'. You meet it most often in a set with its opposites: atheism is the belief that there is no god, and agnosticism is the position that we cannot know either way. In narrower philosophical writing, theism often means something more specific still — belief in one god who made the world and still acts in it — which is why writers separate it from deism, the belief in a god who made the world and then left it alone.

Grammar note: like other -ism words, it normally takes no article. Say 'theism holds that…', not 'the theism'. The person is a theist, and the adjective is theistic. Be careful with the pronunciation — the first part sounds like 'thee', so it is THEE-ism, three syllables, not 'tay-ism'.