The Earth rotates on its axis. What is an axis?

In general, an axis is any line that helps describe how some objects move. Imagine a string that goes right through the center of a ball (or any solid object). Pull the string taught and imagine spinning the object on the string. The string represents the part of the ball that is not really moving, or that is moving the least. In the case of objects moving in space, there is no physical string or other structure. The earth naturally moves in a spinning fashion, around an imaginary line called the axis. In the case of a freely spinning body, the axis will contain the point known as the center of gravity.