Glossary
 Axis
 Camouflage
 Chameleon
 Equation
 Horizontal
 Intercept
 Intersect
 Line
 Line Segment
 Origin
 Parallel
 Perpendicular
 Point
 Slope
 Undefined
 Vertical
Axis
An axis is a line that we use to find points and other lines. If we want to talk about more than one axis, we say "axes."
In the plane, we use two axes called the xaxis and the yaxis. Here is a picture:
The xaxis has equation y = 0, and the yaxis has equation x = 0.
Camouflage
When you camouflage yourself, you change to match your surroundings. For example, you might cover your face with mud and put leaves in your hair so that you could hide in a forest. Soldiers often wear clothing that camouflages them.
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Chameleon
A chameleon is a type of lizard. Chameleons have long, sticky tongues for catching bugs. They can change color to warm up or cool off, and to camouflage themselves. Chameleons also change color when they are angry or scared.
Chameleons can move one eye without moving the other. This makes it easy for them to keep track of their surroundings.
Equation
An equation shows that two things are equal. For example, the equation x = 8 tells you that x and 8 have the same value.
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Horizontal
A horizontal line is completely flat. This line is horizontal:
Intercept
A line's xintercept is the place that it crosses the xaxis, and its yintercept is the place that it crosses the yaxis. You might think of the line catching an axis, as if it were intercepting a ball.
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Intersect
When two lines cross each other, we say that they intersect. This is the same idea as the intersection of two roads.
Line
In geometry, lines are always straight. Lines are onedimensional, and they go on forever in both directions. Here is a picture of a line:
(The arrows are supposed to show that the line continues forever.)
Line Segment
Part of a line is called a line segment. Here is a picture:
The dots stand for the points at each end of the segment. If the points at each end aren't included, we use circles instead of dots:
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Origin
The origin is the point (0, 0).
Parallel
Two lines are parallel if they never meet. Here is a picture of two parallel lines:
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Perpendicular
Two lines are perpendicular if their intersection has a 90° angle, like the corner of a square. These two lines are perpendicular:
Point
A point is one location in space. Points are zerodimensional.
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Slope
Slope is a number that shows how steeply a line slants. We often use the letter m to stand for a line's slope.
Undefined
In math, we call something undefined when we can't think of a description that makes sense. For example, 6/2 is a number that you can multiply by two to get six:
This means 6/0 ought to be a number that you can multiply by zero to get six. But zero multiplied by anything is zero, not six. We can't pick a good number for 6/0, so we say that it is undefined. (For more, see Dividing by Zero from the Ask Dr. Math FAQ.)
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Vertical
Vertical lines point straight up and down. This line is vertical:
A vertical line always has the same xcoordinate, which we'll call a. This means that the line's equation is x = a.
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