amplitude
Amplitude is how big a wave swings, the height of its hills above the calm middle line. A loud sound or a strong signal has a large amplitude; a faint one has a small amplitude.
For a sine wave the amplitude usually means the peak value Vp, the maximum distance from the centre (zero) line to the crest. A signal v(t) = 5 times sin(...) has an amplitude of 5 V: it reaches +5 V at the top and -5 V at the bottom. Do not confuse amplitude (centre-to-peak) with peak-to-peak (bottom-to-top), which is twice as big, or with RMS, which is smaller.
Amplitude carries the strength of a signal, the volume of audio, or the size of a sensor reading. Caveat: amplitude is used loosely. People sometimes quote peak, sometimes peak-to-peak, sometimes RMS, so always check which one a number means before you trust it; mixing them up is a classic source of factor-of-2 (or factor-of-2.8) errors.
Turning up a guitar amp's volume increases the signal's amplitude (taller waves) while leaving its frequency (pitch) unchanged.
Amplitude is loudness; frequency is pitch.
The word amplitude is ambiguous in casual use. Confirm whether a quoted figure is peak, peak-to-peak, or RMS before doing any math with it.