Short answers, pulled from the story.
Timbre is the quality of a musical sound that distinguishes it from other sounds at the same pitch and loudness. It is governed primarily by frequency spectrum and envelope, and it allows listeners to tell apart instruments such as a guitar and a piano playing the same note.
Timbre is governed primarily by frequency spectrum and envelope. The envelope includes attack time, decay, sustain, and release (ADSR), and the frequency spectrum includes the balance of harmonics and overtones present in the sound.
The ASA Acoustical Terminology definition 12.09 describes timbre as "that attribute of auditory sensation which enables a listener to judge that two nonidentical sounds, similarly presented and having the same loudness and pitch, are dissimilar." It adds that timbre depends primarily on frequency spectrum but also on sound pressure and temporal characteristics.
Wagner used timbre as a structural tool in his orchestration. His "Sleep motif" from Act 3 of Die Walküre features a descending chromatic scale that passes through woodwind, massed strings, and brass, using shifts in instrumental color as part of the composition's architecture.
The tristimulus timbre model is borrowed by analogy from color theory. It reduces the harmonics of a sound to three grouped values: the weight of the first harmonic, the combined weight of the second through fourth harmonics, and the combined weight of all remaining harmonics. Further studies are needed to validate it.
Musicians can alter timbre through playing technique and electronic effects. A violinist can produce a light, airy timbre by playing sul tasto or a harsh, aggressive timbre by playing sul ponticello. Electric guitar and electric piano players can also modify timbre using effects units and graphic equalizers.