In written music, the gradation of volume in music. Long before we could actually measure the volume of sound, music masters defined the overall volume of a passage of music with written indications on sheet music. The following are the most commonly used dynamics:

ff (Fortissimo) – Very loud, or very strong.
f (Forte) – Loud, or strong.
mf (Mezzo forte) – Medium loud, medium strong or “half loud.”
p (Piano) – Soft or quiet.
pp (Pianissimo) – Very soft or very quiet.

In music and sound recording, dynamics also refers to the dynamic range of the material. Audio passages with a large dynamic range are sometimes said to have a lot of dynamics. This is subjective and depends, to some extent, on the span of time between the loudest and softest passages.

Dynamics also refers to the general category of audio equipment designed to modify the dynamic range of audio in some capacity. This would include things like compressors, limiters, expanders, gates, etc. These are sometimes known as dynamics processors.