• ### Centroid

The point in a triangle at which the medians of a triangle intersect.

• ### Circumcenter

The point at which the perpendicular bisectors of a triangle intersect.

• ### Concurrent

Intersecting at one point; lines, rays, segments, etc. are concurrent when they intersect at one point.

• ### External Segment

The segment contained by a secant segment with an endpoint on the circle and at the fixed point outside the circle whose points all lie outside the circle (except the endpoint on the circle).

• ### Incircle

The point in a triangle at which the angle bisectors of a triangle intersect. This point is also the center of a circle inscribed in the triangle.

• ### Inscribed Angle

An angle whose vertex lies on a circle and whose sides are contained by secant lines.

• ### Internal Segment

The segment contained by a secant segment whose endpoints are both on the circle.

• ### Isosceles Trapezoid

A trapezoid with congruent legs.

• ### Lower Base Angles

The angles in an isosceles trapezoid whose vertices are the endpoints of the longer base.

• ### Median of a Triangle

A segment within a triangle with one endpoint at a vertex of the triangle and the other endpoint at the midpoint of the side opposite the vertex. Every triangle has three medians.

• ### Midsegment

A segment within a triangle whose endpoints are midpoints of the sides of the triangle. Every triangle has three midsegments.

• ### Orthocenter

The point at which the altitudes of a triangle intersect.

• ### Point of Concurrency

The intersection point of concurrent lines, segments, etc.

• ### Remote Interior Angles

The two angles of a triangle that are not adjacent to the exterior angle which is drawn by extending one of the sides.

• ### Secant Segment

A segment with one endpoint on a circle, the other endpoint at a fixed point outside the circle, and one point of intersection with the circle, not including its endpoint.

• ### Theorem

A statement about geometric figures that has been proved in the past, and can be accepted as a truth in the present without proof. A list of important theorem's can be found in review.

• ### Upper Base Angles

The two angles of an isosceles trapezoid whose vertices are the endpoints of the smaller base.