Timbre
Definition
The characteristic quality of a sound that distinguishes it from another sound at the same pitch and volume. Determined by the harmonic content, envelope, and spectral distribution of a sound. Timbre is what makes a piano sound different from a guitar playing the same note at the same loudness.
In Simple Terms
The unique "color" or "texture" of a sound — what makes a trumpet sound like a trumpet and not a violin, even when they play the same note at the same volume. Every instrument, every voice, every sound has its own timbre. EQ, saturation, and effects all shape timbre.
In Practice
A vocalist and a flute both perform the note A4 at 440 Hz at the same volume. Despite identical pitch and loudness, the sounds are instantly distinguishable because their harmonic content — the pattern of overtones above the fundamental — is completely different. That difference is timbre.