# Geometry Terms

### 52 terms by ProfKersplat

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### Point

A specific place, marked by a dot and usually named with a letter.

### Line

A straight path that goes on forever in both directions. Named using two points on the line. A line has arrows on both ends that show it keeps going.

### Line Segment

A part of a line that has two endpoints. A line segment is named using the two endpoints.

### Ray

A ray has an endpoint on one side and keeps going forever on the other. You always name a Ray using the endpoint first.

### plane

A flat surface that keeps spreading in all directions forever. Must be named by three points on the plane.

### Intersecting lines

Two lines that cross

### Parallel lines

Two lines, that lie next to each other without ever getting close together or further apart.

### Perpendicular lines

Two lines that make right angles when they cross.

### degrees

The units used to measure an angle.

### Right angle

An angle that measures 90 degrees.

### Acute angle

An angle that measures less than 90 degrees. (Babies are small, babies are cute. An acute angle is small like a baby)

### Obtuse angle

An angle that is more than 90 degrees. (Obtuse can also mean slow. An obtuse angle is a big, slow angle.)

### Straight angle

An angle that measures 180 degrees. This is a flat angle. It looks like a line.

### vertex

The vertex is the point where two rays or line segments meet to form an angle.

### Equilateral Triangle

A triangle that has all three sides the same length.

### Isosceles Triangle

A triangle with two sides that have the same length.

### Scalene Triangle

A triangle with no equal length sides.

### Right Triangle

A triangle that has a 90 degree, or right, angle.

### Acute Triangle

A triangle that has three interior angles that are less than 90 degrees.

### Obtuse Triangle

A triangle that has one interior angle that is greater than 90 degrees.

### Interior Angle

An angle found on the inside of a shape or object.

### Exterior Angle

An angle found on the outside of a shape or object.

### Congruent

Two shapes are congruent when all of the sides and angles of the two shapes are exactly the same. The object can be rotated and face a different direction, but the measurements must be the same.

### Similar

The angles and the overall appearance of the shapes are the same, but one object is larger than the other.

Any shape that is a polygon and has four sides.

### Polygon

Any object made of at least three straight lines. There can be no gaps, no curves, and no lines that cross each other.

### Rectangle

Opposite sides are parallel and congruent. Shape has four right angles.

### Square

All four sides are congruent and parallel. There are four right angles.

### parallelogram

Four sided shape. Opposite sides are congruent and parallel.

### rhombus

A four sided shape with all sides congruent and opposite sides parallel.

### trapezoid

A four sided shape with only one set of parallel lines.

### Regular Polygon

A shape with congruent sides and identical angles.

### Irregular Polygon

A shape made up of sides that are different links and unlike angles.

### Pentagon

Five sided shape.

Six sided shape.

### Heptagon

Seven sided shape.

### Octagon

Eight sided shape.

### Nonagon

Nine sided shape.

Ten sided shape.

### Undecagon

Eleven sided shape.

### Dodecagon

Twelve sided shape.

### Diagonal

A line segment that joins two vertices of a shape by going through the middle of the shape.

### Center (of a circle)

A point in the middle of a circle that is the same distance from outside of the circle all the way around.

A line segment that connects the center of the circle to a point on the circle.

### diameter

A line segment that goes from one side of the circle to the other and passes through the center of the circle.

### Chord

A line segment that goes from one side of the circle to the other side. A chord does not have to go through the center.

### Central Angle

An angle, whose vertex is at the center of a circle.

### Line Symmetry

If an object can have a line drawn down the center and both sides are congruent, then the object has line symmetry.

### Rotational Symmetry

If you can rotate an object less than 360 degrees and the object looks like it did before you turned it, then the object has rotational symmetry.

### Circumference

The distance around the outside of a circle

### Complementary Angle

two angles that add up to 90 degrees

### Supplementary Angle

two angles whose measures have a sum of 180 degrees

Example: