(dense astronomical body such as white dwarf or denser)
A compact object (or compact star, sometimes abbreviated CO)
is a general term for small, very massive stellar objects,
typically meant to include
white dwarfs, neutron stars, black holes,
and any other hypothesized very dense exotic stars.
They are typically stellar remnants,
what's left after a star's fusion has run out.
The term degenerate star is also used for compact objects that
are not black holes.
Some references use the term compact object for a
black hole or neutron star, but not for a white dwarf.
A compact object merger is a merger of two such stars,
with a luminosity potentially 1015 times that of the Sun,
and sometimes a source of detectable gravitational waves.
The term central compact object (CCO) is used for a compact object
centered within some observed larger object
such as a planetary nebula or supernova remnant.