Final answer:
Charlie Parker, nicknamed "Bird", was the alto saxophonist credited with defining the harmonic language of bebop.
Step-by-step explanation:
The alto saxophonist nicknamed "Bird" who was credited with defining the harmonic language of bebop was Charlie Parker. Parker was a prominent figure in the development of bebop in the 1940s, along with other musicians like Dizzy Gillespie and Thelonious Monk. He revolutionized jazz improvisation with his virtuosic playing, complex melodies, and innovative use of harmony.