In a simple pendulum, amplitude is the maximum displacement from the equilibrium position. It describes the greatest distance the bob moves away from its central, resting line during an oscillation.
For a pendulum, equilibrium is the lowest point of its swing, where gravity pulls the bob downward along the supporting string. If the bob is released from one side, the distance or angular displacement between that release point and the vertical center is the amplitude. The bob then passes through equilibrium and reaches an equal displacement on the opposite side, assuming negligible energy loss.
Amplitude is not the same as period, which is the time for one complete swing cycle, or frequency, which counts cycles per second. Wavelength applies primarily to waves, not to the back-and-forth motion of a single pendulum. For small swings, a simple pendulum's period depends mainly on its length and local gravity, not on its amplitude; at larger angles, the period increases slightly as the swing becomes less ideal.
Over time, air resistance and friction reduce the amplitude, producing a damped oscillation.