Amplitude

The maximum displacement of a wave* form measured relative to the resting position of the medium through which the wave is moving. Amplitude is measured in the orientation of particle movement. In longitudinal waves the motion is in the same orientation as energy propagation. In transverse waves, it is perpendicular to energy propagation.  

Pendulums: The amplitude of pendulum is the distance between the bob's resting position and its position at maximum displacement. 

Physical wave*s: displacement of physical waves Physical waves is often described using wave height instead of amplitude.  A wave's height is equal to two times its amplitude.