An "axis" is an imaginary line around which an object rotates. In geometry, it can refer to a straight line that divides a shape into two symmetrical parts. For example, the x-axis and y-axis in a coordinate system help define the position of points in a two-dimensional space. In the context of Earth, the axis is the line that runs from the North Pole to the South Pole. This axis is tilted at an angle of about 23.5 degrees, which is responsible for the changing seasons as the Earth orbits the Sun.