An Etymological Dictionary of Astronomy and Astrophysics

For a → triangle with two sides extended in the direction opposite their common → vertex,
a circle that lies outside the triangle and is tangent to the three sides (two of them extended). The center of the excircle, called the → excenter, is the point of intersection of the bisector of the interior angle and the bisector of the exterior angles at the other two vertices.

See also: → ex-; → circle.