Synthetic food was any type of food which was not naturally occurring.

In 2266, Doctor Thomas Leighton used the ruse of having discovered a new synthetic food to lure the USS Enterprise to Planet Q. (TOS: "The Conscience of the King")

In ultimately unused dialogue from the final revised draft script of "The Conscience of the King", Spock asked Kirk, following the Enterprise's visit to Planet Q, about the synthetic food samples they had intended to pick up from Dr. Leighton, so Kirk broke the news to Spock that there weren't actually any, a fact Spock said would have to be noted.