Timbre (pronounced 'tam-ber') is why a violin and a flute sound different playing the same note at the same volume. It is determined by: the harmonic spectrum (relative levels of fundamental and overtones), the attack characteristics (how the sound starts), and the formant structure (fixed resonant frequencies of the instrument body).
In audio production, preserving natural timbre is a primary goal of high-quality recording chains. Poor microphones, preamps, or converters alter timbre by adding or removing harmonics. EQ can reshape timbre — boosting 3 kHz adds 'presence' and brings a vocal forward; cutting 400 Hz removes 'boxiness.'