For our purposes the word mythology has two related meanings. Firstly it refers to a collection of myths that together form a mythological system. Thus one can speak of "Egyptian Mythology", "Indian Mythology", "Maori Mythology" or "Greek Mythology". In this sense one is describing a system of myths which were used by a particular society at some particular time in human history. It is also possible to group mythologies in other ways. For example one can group them geographically and then speak of "Oceanic Mythology", "Oriental Mythology" and "African Mythology".
A second meaning of the term mythology is the academic study of myths and systems of myths in general.