Polytheism is a belief in and/or worship of multiple gods and goddess'. The word comes from the Greek, "Poly" which means many and "Theoi" which means gods. You have Soft Polytheism and Hard Polytheism. Many modern day Wiccans are Soft Polytheists, which means that believe in many gods but believe them to be aspects of The God or The Goddess. Whereas a Hard Polytheist, such as an Asatruar, believes each god and goddess is an individual entity, with their own personality and character and is not part of a greater whole.
Robin doesn't believe in monotheism, she worships many gods and therefore believes in polytheism.