In simple terms, timbre is what makes a particular musical instrument or human voice have a different sound from another, even when they play or sing the same note. For instance, it is the difference in sound between a guitar and a piano playing the same note at the same volume.
Timbre in modern English generally refers to the quality of a sound made by a particular voice or musical instrument; timbre is useful in being distinct from pitch, intensity, and loudness as a descriptor of sound.
Timbre is a basic concept in music that affects everything from sound design to arrangement and mixing. When trying to describe timbre’s meaning in music, it can be tough to know where to start.
Timbre is also known as tone quality, tone color, or voice. Some of the many words used to describe the timbre of instruments include rich, bright, mellow, dark, buzzy, and warm. The timbre also includes the way the sound changes over time.
Timbre, quality of auditory sensations produced by the tone of a sound wave. The timbre of a sound depends on its wave form, which varies with the number of overtones, or harmonics, that are present, their frequencies, and their relative intensities.
Describing qualities of sound (Definition of timbre from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
Timbre (tone color or tone quality) is what differentiates two sounds of the same frequency (same note). For example, the C note played on the guitar sounds very different from the C note played on the piano or flute.
Essentially, timbre is a word to describe the overall sound of a note, and it’s how we describe why one instrument sounds different from another, even when playing the same note.