diner vs tablemate

diner and tablemate both are nouns.

Word	Noun	Adjective	Verb	Adverb
diner	Yes	No	No	No
tablemate	Yes	No	No	No

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

Other hyponyms of diner include carver, cutter, dining companion.

diner (noun)	tablemate (noun)
a passenger car where food is served in transit	someone you dine with
a restaurant that resembles a dining car
a person eating a meal (especially in a restaurant)