A two-dimensional shape with nine straight sides is called a nonagon. This nine-sided polygon, which also has nine edges or vertices, is also called an enneagon. The term enneagon is preferable, but less popular than the name nonagon because the latter is simpler to spell and pronounce.
The name enneagon is derived from the Greek word ?enneagonon,? which is a combination of two terms that mean nine and corner. The name nonagon, in contrast, has a mixed Roman and Greek origin. It is from the Latin word ?nonus,? which means ninth, added to the suffix ?gonon.? Both enneagon and nonagon are accepted terms to refer to a shape with nine sides.