Consider a spinning ball. Every point on it is moving, except the ones right in the middle: these form a line of stillness around which the rest of the ball spins. This line is called the axis of rotation.
More precisely, the axis of rotation of a rotating object over a period of time is a point or a line connecting points that do not change position while that object rotates, drawn when the observer assumes he/she does not change position relative to that object over time.
Conventionally, the direction of the axis of rotation is such that, if you place your eye looking in that direction, the rotation appears clockwise, as illustrated below, where the yellow arrow shows the rotational movement, while the purple one shows the rotation axis:
To remember this convention, hold your right hand in a thumbs-up gesture: 
If the rotation follows the direction of the curled-up fingers, then the direction of the axis of rotation is considered to be the same as the direction which the thumb is pointing in.
If the rotation follows the direction of the curled-up fingers, then the direction of the axis of rotation is considered to be the same as the direction which the thumb is pointing in.
This gesture is a different form of the right-hand rule and is sometimes called the right-hand grip rule, the corkscrew-rule or the right-hand thumb rule. From now on in this book, it will be referred to as 'the right-hand grip rule'.
When describing the direction of a rotating object, do not say that it rotates left-to-right/clockwise, or right-to-left/counterclockwise. Each of these claims on their own are useless, because they're relative to the observer. Instead of saying this, find the direction of the axis of rotation and draw an arrow to represent it. Then people who know the right-hand grip rule will be able to figure out what the direction of rotation of the object is, by using the rule when interpreting your drawing.
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