Monotheism is the belief in a single god. It is usually contrasted with polytheism, belief in many gods. The world's major monotheistic religions (in chronological order of founding) include Judaism, Christianity and Islam.
"[The other Gods of polytheism] —LAUGHED themselves to death once on a time! That took place when the unGodliest utterance came from a God himself—the utterance: 'There is but one God! Thou shalt have no other Gods before me!'"