Monotheism is the belief in the existence of one God. The primary alternatives to monotheism are polytheism - belief in many gods, atheism - belief in no gods, or some kind of variation on pantheism - the belief that the whole universe is God. Judaism, Christianity and Islam are all monotheistic, although it is possible to challenge whether or not the Christian concept of the Trinity is really monotheistic.
Monotheisms tend to have evolved out of polytheisms: if one looks at the Old Testament, one finds a large variety of rival Gods which aren't denied in the same way that an modern Jew or Christian would deny, say, a Hindu God - no, they are said to exist in some sense, but the worship of them is forbidden (henotheism or monolatrism). It is Catholic dogma that monotheism, not polytheism, has been around since it was revealed to the earliest man.