From The Art and Popular Culture Encyclopedia
In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments. The physical characteristics of sound that mediate the perception of timbre include spectrum and envelope. In psychoacoustics, timbre is also called tone quality and tone color.
In simplest terms, timbre is what makes a particular sound unique from another sound. For instance, it is the difference between a guitar and a piano playing the same note at the same volume.