Complex sentence: Both "indie" and "indie pop" had originally referred to the same thing during the late 1970s, the concept of "indie music" did not crystalize until the late 1980s and early 1990s.
Split into simpler sentences: Both "indie" and "indie pop" had originally referred to the same thing during the late 1970s. The concept of "indie music" did not crystalize until the late 1980s and early 1990s.