side order vs side dish

side order and side dish both are nouns.

Word	Noun	Adjective	Verb	Adverb
side order	Yes	No	No	No
side dish	Yes	No	No	No

As nouns, side order and side dish are synonyms defined as:

side order and side dish: a dish that is served with, but is subordinate to, a main course

Other synonyms of side order include entremets.

side order (noun)	side dish (noun)
a dish that is served with, but is subordinate to, a main course	a dish that is served with, but is subordinate to, a main course