Collapsar is the shortened form to describe a collapsed star.
When an old star no longer has enough fuel for significant fusion reactions there can be three possible outcomes depending on the star's mass. If the star’s mass is less than 1.4 solar masses, known as the Chandrasekhar limit, it will stabilize and shrink, becoming a white dwarf. Above the said limit the star will either become a neutron star or a black hole.