What is the minimum initial mass required for a star to be classified as massive?

Exceeds about eight times the mass of the Sun.

The distinction between stars that meet a gentle end and those that undergo catastrophic collapse is based primarily on their initial mass relative to our Sun. Stars that possess an initial mass significantly greater than the Sun are designated as massive stars. Specifically, the text defines a star as generally massive if its initial mass exceeds a threshold of about eight times the mass of the Sun. Stars below this mass threshold typically end their lives more tranquilly by puffing off their layers to become planetary nebulae surrounding a white dwarf, whereas these high-mass stars proceed to fuse elements up to iron, leading to supernova.