Most of the oscillations that occur in nature are subject to resistive forces. Such oscillations are known as damped oscillations.
For the case of a mechanical oscillator (e.g. simple pendulum), these forces act in the direction opposite to the velocity of the oscillator. The movement of the oscillator is constrained by the resistive forces.
The total energy of a damped oscillator decreases over time. This means that the amplitude of oscillation decreases over time depending on the strength of the resistive force.
The process of applying a resistive force to a free oscillator is called damping. There are three main types of damping arranged according to increasing damping strength: light, critical and heavy damping.