In geometry, a nonagon (or enneagon) is a nine-sided polygon. The name "nonagon" is a hybrid formation, from Latin (nonus, "ninth" + gonon), used equivalently, attested already in the 16th century in French nonogone and in English from the 17th century. The name "enneagon" comes from Greek enneagonon, (εννεα, nine + γωνον (from γωνία = corner)), and is arguably more correct, though somewhat less common. A regular nonagon has internal angles of 140°. The area of a regular nonagon of side length a is given by