A seven-sided polygon is called a "heptagon." "Hepta-" or "hept-" is a prefix from the Greek word meaning "seven." In addition to having seven sides, the heptagon also has seven angles where the sides meet.
The seven-sided figure is also known as a "septagon." However, this name is a combination of a Latin prefix, "septa-," with a Greek suffix, "-gon." Linguistics experts recommend use of "heptagon" as the name for this type of figure. In a regular heptagon, each of the interior angles is equal to 51.43 degrees. The word "heptagon" is rare in literature and use outside of mathematics.