A Abstract being is a entity that is the living embodiment of some type of existential concept like Death itself is death embodied and so it will have powers and abilities that often represents around the said concept that they embodied. Such entities are almost unkillable since they cant be truly destroyed unless either the concept they embody is completely erased from existence itself or if the highest being wiped them from existence.
Example
The Living Tribunal,Mistress Death,Eternity, and Infinity are all Abstract beings.