We generally characterize sounds by their pitch, loudness, and quality. We refer to the quality of a sound as its timbre. Timbre (tam-burr) is the color or character of a sound. We hear timbre because our ears fuse together and experience as a single sound the multiple frequencies that most sounds contain. More simply, timbre is the sound characteristic that allows our ears to distinguish between sounds that have the same pitch and loudness.
Timbre is mostly determined by a sound’s harmonic content it its dynamic characteristics such as vibrato and the attack-delay envelope of the sound. (Georgia State University: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/timbre.html)