In physics, a damper is a device that associates each force to a particular velocity.
The velocity of a damper is defined as the rate at which the damper is lengthening.
The force of a damper is defined as the force applied TO the damper by something else.
A common damper consists of an oil-filled tube, with a plunger that has to squish its way through the oil as you lengthen or shorten the damper. The more force you apply to stretch the damper, the faster the plunger moves through the oil. If you stop trying to stretch the damper (you stop applying a force to it), it immediately stops lengthening; its velocity drops immediately to zero.
The force is always the same at both ends of a damper (see: through variable) If the force is positive, the damper is in tension. A negative force means that the damper is in compression.
Likewise, the velocity is positive when the damper is lengthening, negative when it is shortening.
The constitutive equation for a damper is F=bV, where b is the dampening constant. The larger b is, the more resistant the damper is.