Appetite was an individual's desire for food.
In 2254, Christopher Pike asked The Keeper why, instead of threatening him with the experience of hell to force him to eat, the Talosian couldn't simply put irresistible hunger in his mind. (TOS: "The Cage", "The Menagerie, Part II")
In 2266, when asked by Captain Kirk why Spock hadn't shown signs of infection from an unknown illness, Doctor McCoy suggested that maybe "the little bugs or whatever they are have no appetite for green blood" (TOS: "Miri")
Lack of appetite could be a marker of illness. In 2364, the Aldeans' symptoms included lack of appetite. (TNG: "When The Bough Breaks")
Pregnancy could increase one's appetite. In 2370, Data reported that a pregnant Spot's appetite had increased by seven percent. (TNG: "Genesis")