Generally, amplitude is the measure of change in an oscillating variable.

In video specifically, “peak-to-peak” amplitude is most commonly used. It is the measure, expressed in Volts or IRE of the difference between lowest and highest voltages over time. Peak-to-peak amplitudes can be measured by viewing the video waveform on an oscilloscope or waveform monitor where voltage is displayed along a vertical “Y” axis, as it changes over time represented along the horizontal “X” axis. The peak negative and positive values of the waveform are easily identified and measured against a graticule. The difference between these peaks is peak-to-peak amplitude.

For RS-170, NTSC, CCIR, and PAL analog composite video, peak-to-peak, amplitude is commonly specified as 1 Vp-p. This is a nominal specification only and should not be taken to mean that amplitude is always 1 Vp-p. Many cameras exceed 1Vp-p for representing bright pictures. Dark pictures may hover around 0.5 Vp-p or less. Equipment that superimposes lines, circles, and other patterns onto video may require the signal to exceed 1 Vp-p.

Mentioned in:
Video False Colorizer & Video Color Synthesizer Model 606S
IRE, defined in terms of video