acute triangle

A triangle containing three angles that are all less than 90 degrees.

adjacent angles

Two angles that share the same vertex and one side. The two sides that are not shared form a larger angle.

altitude

In a triangle, a line drawn perpendicular to the base, from the base to the opposite vertex, that represents the height of the triangle.

angle

A shape formed by two rays or line segments sharing a common endpoint or two lines that intersect. An angle has one vertex and two sides.

area

The size of the space contained within an enclosed twodimensional figure. Area is typically measured in square units such as square inches or square centimeters.

base

In an isosceles triangle, the one side that is not equal to the other sides.

base angle

In an isosceles triangle, one of the two angles formed by the base and a leg.

congruent

Equal or similar to each other.

equilateral triangle

A triangle containing three equal sides that are the same length.

interior angle

An angle located within a closed figure. A triangle has three interior angles.

isosceles triangle

A triangle containing two equal sides that are the same length.

leg

In an isosceles triangle, one of the two sides that are equal in length.

obtuse triangle

A triangle containing one angle that is greater than 90 degrees. The other two angles must total less than 90 degrees.

perpendicular

Forming a 90° right angle.

perpendicular lines

Two lines that intersect to form a 90° angle.

right angle

An angle that measures exactly 90 degrees.

right triangle

A triangle containing exactly one 90° angle. The other two angles must total exactly 90 degrees.

scalene triangle

A triangle containing three unequal sides that are all different lengths.

supplementary angle

Two angles that, when added together, equal 180 degrees.

triangle

A closed figure with exactly three sides. The three sides meet to form three interior angles.

vertex

A point where two lines or line segments meet or intersect. A triangle has three vertices.

vertex angle

In an isosceles triangle, the angle formed by the two legs of equal length. The vertex angle is always opposite the base.

vertical angles

Two angles that share the same vertex and are positioned directly opposite one another. Vertical angles are formed whenever two lines intersect.

vertices

The plural term for vertex.
