Polytheism is the belief in more than one god.
It is worth noting that polytheism is the oldest form of religion and the vast majority of religions are polytheistic in nature. It has been argued that Christianity itself could be classed as polytheistic in nature as Christians belief in the Father, Son and Holy Ghost, however Christians view these as one God.
Islam is strictly monotheistic (the belief in a single god), whilst Judaism is traditionally monotheistic with period of polytheism in its' past. Both Archaeology and even the Bible itself supports the idea that the Jewish god was worshipped alongside other gods such as Asherah for instance during certain periods in the past.