Parts of the Wave 

Equilibrium 

Equilibrium:  In the diagram below the white line represents the position of the medium when no wave is present. This medium could be imagined as a rope fixed at one end a few feet above the ground and held by you at the other end.

The yellow line represents the position of the medium as a wave travels through it. We simply say that the yellow line is the wave. If we consider the rope mentioned before, this wave could be created by vertically shaking the end of the rope.

Crest and Trough

 Crest and Trough:  The section of the wave that rises above the undisturbed position is called the crest. That section which lies below the undisturbed position is called the trough. These sections are labeled in the diagram below:

Wavelenght

 Wavelength:  The wavelength of a wave is the distance between any two adjacent corresponding locations on the wave train. This distance is usually measured in one of three ways:  Crest to the next crest, trough to next trough, or from the start of a wave cycle to the next starting point. This is shown in the following diagram:

Amplitude 

Amplitude:  The term amplitude can have slightly different meanings depending upon the context of the situation.

Its most general definition is that the amplitude is the maximum positive displacement from the undisturbed position of the medium to the top of a crest. In some discussions it is important to distinguish between positive and negative amplitudes. These displacements are shown in the following diagram: