Timbre has generally been used without any great consistency of meaning to refer very loosely to the 'colour' of a sound. It has often, and incorrectly, been treated as a discrete 'parameter' of sound, along with other facets such as frequency and pitch. Creative practice and research in electroacoustic music have revealed the unsatisfactory nature of this imprecision, and opened up the notion to a much more complex and protean situation, where timbre exists in fragile relationships and continua with frequency, spectral content, sonic identity, and source recognition. This leads to a situation where, in many musical examples, it is hard to separate timbre from the overall musical discourse.
A general, sonic physiognomy whose spectromorphological ensemble permits the attribution of an identity. (Source - Denis Smalley (1994). Defining Timbre - Refining Timbre. Contemporary Music Review Vol. 10, Part 2. London: Harwood Academic Publishers.)
Ensemble of the parameters of pitch, duration, amplitude, spectral components, dynamic evolution, etc. which determine the colour of a sound. (Source - Dictionnaire des arts médiatiques: http://www.dictionnairegram.org)