Amplitude
Definition
How big a wave's oscillations are — measured from the middle line to the top of a crest.
Symbol: · Unit: depends on the wave — meters for a water wave, pascals for sound pressure
Technical definition
The maximum displacement of a wave from its equilibrium position. Wave energy is proportional to the square of the amplitude, so small amplitude changes mean large energy changes.
Example
Shout and whisper the same note: the frequency (pitch) is identical, but the shout's pressure wave has a far larger amplitude — which is why it carries more energy and sounds louder.
Amplitude is not wavelength or frequency. Doubling a sound wave's amplitude makes it louder (four times the energy, since energy goes as A²), not higher-pitched. Pitch comes from frequency; loudness comes from amplitude — mixing them up is the single most common wave mistake, and it breaks every question about wave energy.