domain vs earldom

domain and earldom both are nouns.

Word	Noun	Adjective	Verb	Adverb
domain	Yes	No	No	No
earldom	Yes	No	No	No

As nouns, earldom is a hyponym of domain; that is, earldom is a word with a more specific, narrower meaning than domain:

domain (noun)	earldom (noun)
the content of a particular field of knowledge	the domain controlled by an earl or count or countess
people in general; especially a distinctive group of people with some shared interest	the dignity or rank or position of an earl or countess
(mathematics) the set of values of the independent variable for which a function is defined
territory over which rule or control is exercised
a particular environment or walk of life