means The distinctive quality or color of a sound that lets you tell one voice or instrument from another, even when they play the exact same pitch at the same volume.

from From French timbre, which originally meant a kind of small bell or drum — and earlier still came, through medieval French, from a Greek word for a hand drum or tambourine (tympanon, the same root behind 'tympanum' and 'tambourine'). The sense slid from the instrument that makes a sound, to a bell's characteristic ring, to the characteristic 'ring' of any sound. The same French word also gave us 'timbre' meaning a stamp or seal, which is why a postage stamp and the color of a clarinet are oddly distant cousins.