Final answer:
The initial position of particles in a wave before any energy is applied is known as the Equilibrium position. The trough is the lowest point in a wave, and amplitude measures the maximum displacement from the equilibrium position.
Step-by-step explanation:
The position of the particles before any wave energy is applied is called the Equilibrium position. In the context of waves, a trough is the lowermost position of a wave, while the amplitude of the wave (A) is the measure of the maximum displacement of the medium from its equilibrium position. In simple harmonic motion, the equilibrium position is where the object would naturally rest in the absence of force, and it is neither stretched nor compressed.