An "axis" is an imaginary line that something turns around. For example, a wheel has an axis which is the middle of the circle.

The Earth's axis is an imaginary line through the middle of the Earth from North to South. The Earth spins around this line.

"Axis" is a technical and mathematical term, so it's most often used with other technical words. Here are some examples:

• In math, you can talk about a horizontal (side to side) axis or a vertical (up and down) axis.
• The action of turning around an axis is called "rotation around an axis". "Rotation" is a more technical word for "turning in a circle".