Timbre, sometimes called “tone color,” is the acoustical quality of a sound that distinguishes its source from another, regardless of pitch or volume. The concept is most easily grasped through example. A trumpet, violin, electric guitar, boy soprano, koto, car horn, etc., might all produce notes of the same pitch, say a C-sharp, but they will have vastly different timbres. Timbre is the feature of the sound that allows listeners to comprehend the difference. Even instruments or voices of the same type are differentiated by timbre: Lady Gaga and Cecilia Bartoli might both sing the same pitch at the same volume level, but listeners will be unlikely to confuse the two singers. 

Timbre rarely plays a significant role in music copyright infringement disputes because timbre alone, unlike melody, is not considered a copyrightable musical element.  Use of different timbres may involve creativity, but does not achieve the level of original expression that copyright is intended to protect.  This was an important consideration in the case of Newton v. Diamond (2003).