What is the definition of timbre in music?

I - The unique tonal quality of a sound that allows different sound sources to be distinguished.

II - The amplitude of the sound wave and its relation to the perception of sound as loud or soft.

III - The perception of sound as high, medium, or low, directly related to the frequency of the sound wave.

IV - The time that a note or sound persists, determined by the time it takes for the sound wave to complete a cycle.