ἀστήρ

Phenomenologically, stars are dots of light in the sky. Hence, in common language also planets are metaphorically called "stars" because they appear as bright dots.

Astrophysics, however, defines "stars" as balls of hot gas emitting energy that they produce with nuclear fusion. This rather modern definition raises the question whether or not the end stadiums of stars (like white dwarfs and neutron stars) are really "stars" because they fulfill neither of the conditions. However, these objects are usually (i.e. except in the case of permitting novae or supernovae) not observable for the naked eye observer making this discussion irrelevant for antiquity.