### Acute

An adorable little angle. By "little," of course we mean "smaller than 90 degrees in measure."

### Adjacent

The conjoined twins of the geometry world. Adjacent angles always share a vertex and a side.

### Angle

The corner of empty space between two lines, rays, or segments that share a point. Usually, angles are measured in degrees (and most of them have at least a bachelor's degree).

### Bisected

Split in two. We can say that an angle is bisected, a segment is bisected, or that the Dakotas are bisected.

### Bisector

Something that splits another thing in half. Like a ray being an angle bisector, or a point bisecting a line segment, or a chainsaw bisecting a zombie.

### Collinear

Used to describe points that are all on the same line.

### Complementary Angles

Two wrongs don't make a right, but two complementary angles do. They're two angles that add up to 90° exactly.

### Congruent

Exactly equal in measure or identical.

### Coplanar

Used to describe lines or points that are all on the same plane.

### Distance Formula

A formula that calculates distances based on the coordinates we plug into it. It takes the form of this lovely thing:

.

### Length

The distance between two endpoints. Can be expressed in units of millimeters, inches, feet, miles, or light-years—they're all lengths.

### Line

A one-dimensional segment that continues on forever in both directions. Time-consuming to draw, so we use arrows on the ends to symbolize that it never ends.

### Line Segment

Sometimes just called a "segment." It's a finite piece of line between two endpoints.

### Midpoint

A point that bisects a line segment into two congruent segments. It's the halfway point.

### Obtuse

An angle larger than 90° in measure. It has nothing to do with their smarts.

### Origin

The point (0, 0) on the

*x*-

*y* graph. In 3D space, it's (0, 0, 0). For any additional dimensions, just add more zeros.

### Parallel

Two lines that are on the same plane but never intersect. They're always in sight, but never touch…sort of sad, ain't it?

### Perpendicular

Two lines that create a 90° angle when they intersect. Well, actually they create four 90° angles, but who's counting?

### Plane

A "slice" of three-dimensional space. It has length and width, but no depth, like a sheet of paper that stretches out forever in all directions.

### Point

The smallest object…ever. It has no mass, no length, and no size. It describes only a location.

### Ray

A hybrid of a line and a segment. It has one endpoint, but then goes off forever in the other direction. It's like a ray of sunshine that starts at the sun and then continues on forever.

### Right Angle

An angle that's exactly 90°. Naturally, that means any angle that isn't 90° is wrong.

### Straight Angle

An angle that measures 180°. The title a straight line gives itself when it wants to seem more impressive.

### Supplementary Angles

Two angles that add up to 180°. It doesn't matter if they're adjacent or on different planets.

### Vertical Angles

Angles that are opposite each other resulting from two intersecting lines. Vertical angles are

*always* congruent.