timbre
The unique quality or color of a sound that distinguishes one instrument from another.
In Depth
Timbre (pronounced TAM-ber) is what makes a trumpet sound different from a violin playing the same note at the same volume. It is determined by the complex mixture of overtones (harmonics) that accompany the fundamental frequency, plus the instrument's attack, decay, and resonance characteristics.
Every instrument and voice has a unique timbre, shaped by its physical construction, playing technique, and the material it is made from. Orchestration — the art of assigning music to different instruments — is essentially the art of combining timbres. Composers like Ravel, Debussy, and Rimsky-Korsakov were masters of orchestral colour, creating vivid sonic landscapes through their command of timbre.
The reason a violin and a flute sound different playing the same note is timbre — the unique recipe of overtones each instrument produces. No two instruments, even of the same type, have identical timbre.