A general term for aspects of sound other than loudness and pitch. Different vowels, for example, have a different quality of sound.
Ladefoged (Elements) – Chapter 2 – Loudness and pitch
These are perceptual phenomena that relate to physical properties of sound.
Ladefoged (Elements) – Chapter 1 – Sound waves
Very brief, but a reasonable place to start if you have no idea what sound is.
This is the new version. Still under construction.