Conversation so far:
Is Jascha Heifetz is a good violin player?
Many consider him to be the greatest violinist of all time.
What were some famous Jascha Heifetz songs?
Brahms's Violin Concerto with Reiner and the Chicago Symphony Orchestra; the 1957 recording of Tchaikovsky's Violin Concerto; the 1959 recording of Sibelius's Violin Concerto with the Chicago Symphony Orchestra
Did Jascha Heifetz have a favorite technique?
Heifetz preferred to be recorded relatively close to the microphone; as a result, one would perceive a somewhat different tone quality when listening to Heifetz during a concert hall performance.

Question: What is it?
Self-contained rewrite: What is timbre?