An Epithet
is,

A descriptive name or title applied to a person;
as, Ivan The Terrible.  Sometimes the person is also called by the epithet alone, such as
calling Queen Mary, 'Bloody Mary',
or calling  Satan, 'Belial'.
An epithet is also a word or phrase used
to express a characteristic quality
of the thing it is describing;
such as, a verdant lawn;
a brilliant appearance;
a just man.

 

 

The word, 'Epithet' was used on this site in the definition of the word, 'Belial'.