AAS

The Angle Angle Side postulate (often abbreviated as AAS) states that if two angles and the non-included side of one triangle are congruent to two angles and the non-included side of another triangle, then these two triangles are congruent.

Alternate segment theorem

For any circle, the angle formed between the tangent and the chord through the point of contact of the tangent is equal to the angle formed by the chord in the alternate segment.

Altitude

A line segment from a triangle vertex and perpendicular to the opposite base.

Annulus

The ring-shaped region bounded by two concentric circles.

Base

The base of a polygon is the side from which the height is measured.

Bisector

Line that divides something, such as an angle, in two equal parts.

Centroid

The point of intersection of the three medians of a triangle. It divides each median in parts of ratio 1 : 2.

Cevian

A line segment from a triangle vertex to the opposing side.

Chord

A chord is a line segment connecting two points on a circle. Equal-length chords span equal angles in the centre.

Circle

All points in a plane that have a fixed distance (radius) from a fixed point (centre).