Timbre
Definition
The ‘color’ of a sound — what makes a piano and a violin sound different even playing the same note.
Technical definition
The perceptual quality of a sound determined by its harmonic spectrum (relative amplitudes of harmonics), attack/decay envelope, and formants. Same fundamental frequency, different harmonic mix → different timbre.
Example
Middle C on a piano versus a trumpet: both 261.6 Hz fundamental, but the trumpet's stronger upper harmonics give it a brighter, brassier timbre.
Common confusion
Timbre is not pitch or loudness — it is the third dimension of sound. Two sounds can match in pitch and volume yet sound completely different because of timbre.