hors d'oeuvre vs appetizer

hors d'oeuvre and appetizer both are nouns.

Word	Noun	Adjective	Verb	Adverb
hors d'oeuvre	Yes	No	No	No
appetizer	Yes	No	No	No

As nouns, appetizer is a hypernym of hors d'oeuvre; that is, appetizer is a word with a broader meaning than hors d'oeuvre:

hors d'oeuvre: a dish served as an appetizer before the main meal
appetizer: food or drink to stimulate the appetite (usually served before a meal or as the first course)

Other hypernyms of hors d'oeuvre include appetiser, starter.

hors d'oeuvre (noun)	appetizer (noun)
a dish served as an appetizer before the main meal	food or drink to stimulate the appetite (usually served before a meal or as the first course)