The greatest displacement of particles in a medium around their mean location is known as:

The largest displacement of a particle from its mean or original location is defined as the wave's amplitude.

The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position, as defined in physics. It is one-half of the length of the vibration path.

Hence, the maximum displacement suffered by the particles of a medium about their mean position is called amplitude.