What characterizes an object designated as a black dwarf?

Answer

It no longer emits any detectable light or heat

A black dwarf is the theoretical final state of a white dwarf after it has cooled down so much that it ceases emitting any observable light or heat, becoming a cold, dark lump of degenerate matter.

What characterizes an object designated as a black dwarf?
starsstellar evolutionastrophysicswhite dwarfBlack Dwarf