# Important Geometry Terms Explained

## Types of Angles

Angles are one of the most basic concepts in geometry, and there are many geometry terms that refer to them. Acute angles are less than 90 degrees, obtuse angles are more than 90 degrees, and right angles are exactly 90 degrees. If two angles are complementary, they add up to 90 degrees, and if two angles are supplementary, they add up to 180 degrees.

## Types of Triangles

Triangles are defined by the types of angles that make them up. A right triangle has one right angle and two acute angles. An isosceles triangle has two equal angles (and two equal sides as well). An equilateral triangle has three equal angles (and three equal sides as well).

## Types of Lines

There are two main geometry terms referring to types of lines that are important to know: parallel and perpendicular. Parallel lines are lines that will never meet, no matter how far they are extended both ways (like the opposite sides of a square). Perpendicular lines are lines that meet at a 90 degree angle (like adjacent sides of a square).

## Types of Shapes

The study of geometry is often thought of as the study of shapes. Everyone knows the definition of triangle, circle, and square, but what about polygon, octagon, and pentagon? Just break apart the word into prefixes and roots to find that polygon is a shape with many sides, octagon is a shape with eight sides, and pentagon is a shape with five sides.

## Types of Measurements

Some of the most basic geometry terms have to do with measuring parts of a shape. The perimeter of a shape is the distance around the lines that make it up. The area of a shape is the measurement of the space inside of it. The circumference of a shape is essentially the perimeter, but it can only refer to the “perimeter” of a circle.

## Circle Terms

In addition to circumference and area, there are three other main terms that apply solely to circles. The vertex of a circle is the point in the direct center of the circle. The radius of a circle is the distance from the vertex to any point on the circle. The diameter of a circle is the distance from one point on the circle to another, passing through the vertex. The diameter of a circle cuts a circle in half, and it is always twice the length of the radius.