Why is a 7-sided shape called a heptagon and not a septagon?

A 7-sided shape is called a heptagon and not a septagon because the names of polygons are derived from the Greek names for the numbers. The Greek for "seven" is "hepta" - thus a heptagon is a 7-sided figure. If the system for naming polygons had been based on the Latin names for numbers, then a 7-sided polygon might have been called a septagon, since the Latin Word for "seven" is "septem," but Latin is not the basis for these names in English.