The oscillates of a simple pendulum are damped oscillations.
Most of the oscillations in air or
in any medium are damped. When
an oscillation occurs, some kind of
damping force may arise due to
friction or air resistance offered by
the medium. So, a part of the energy
is dissipated in overcoming the
resistive force. Consequently, the
amplitude of oscillation decreases with
time and finally becomes zero. Hence the oscillations of a simple pendulum eventually stops.