A lever, by definition, is a rigid bar that rotates around a fixed point. The fixed point is called a fulcrum. A lever is a simple machine, and its function is to use a force, called effort, to move a weight, called a load or resistance.
There are three classes of levers that share the same basic definition. The difference between classes is where the load and effort are in relation to the fulcrum. In class 1 levers, the fulcrum is between the load and the effort, as in a seesaw. In class 2 levers, the load and effort are both on one side of the fulcrum. In class 3 levers, the load and effort are on opposite sides of the fulcrum; when an arm and tennis racket together constitute a lever, the shoulder is the fulcrum, and the ball is the load the lever's effort is intended to move.