Wave Equilibrium:


Define:

The Net torque, and the net force both equal zero. Equilibrium is like a stable point of a wave, it is neither at its peak being positive, or its trough being negative. The trough is at the lowest point of a wave. The peak is at the top of a wave. The equilibrium is like the middle mark of the wave, or more like a wave medium. So the wave travels from trough to equilibrium to peak and in reverse.


Unlike peak or trough, equilibrium is..... the middle ground, the balance of the wave, where the wave truly travels through. It is the zero mark, its neither negative or positive.


sources:

http://www.merriam-webster.com/dictionary/Equilibrium
http://www.physics247.com/physics-help/equilibrium.shtml
http://www.britannica.com/EBchecked/topic/190745/equilibrium