A side dish, side order, or side, was a food that complemented the main dish.
In 2370, Julian Bashir ordered a side order of zilm'kach from the Klingon restaurant. (DS9: "Melora")
In 2373, Rom ordered "two eggs over easy, three strips of bacon and a side of corned beef hash" from Quark, rather than his usual puree of beetle, the former being Chief Miles O'Brien's typical breakfast. (DS9: "The Assignment")
Replicator entrée 103 consisted of "curried chicken and rice with a side order of carrots". (DS9: "Blaze of Glory")