Amplitude in an oscillating system is the measure of change with each oscillation within the system and is a general indicator for the amount of energy contained in a wave. For example, the amplitude of a sound wave is a measure of its volume.
The amplitude of an oscillating system of symmetrical waveforms is defined as the maximum value of the wave at its peak above zero, or the height of the wave. A symmetrical waveform starts from a zero point and climbs to its maximum, then falls back to zero before repeating in the opposite direction. This maximum, whether it is at the peak or the trough of the wave, is the amplitude. Amplitude is also known as the peak value of a wave, when referring to symmetrical and periodic wave forms.
Other references to amplitude include the root mean square of the wave, or RMS, and peak-to-peak amplitude. The RMS value is essentially an average of the overall energy output of the wave, taking into consideration the rise and fall of the waveform. Peak-to-peak amplitude is the total change in magnitude of the waveform between its maximum peak value and maximum trough value. In most situations, amplitude is understood to mean the peak value of the waveform, unless specifically stated.