Apples are an appetizer. If you added it with something else like peanut butter or even you rosted it a sprinkled cinnamon over, it would be an appetizer.
Definition: A food or drink served usually before a meal to stimulate the appetite.
Technically, anything could be considered an appetizer, so long as the portion served did not do away with your appetite!
Try this, core an apple, and stuff the cavity with white and purple raisins, mixed with cinnamon and brown sugar. Bake this in the oven until the apple is soft. Serve warm, paired with a sweet wine. Heck, could be a dessert as well...
It's a fruit that's used for many meals (snacks, breakfast, with salads, in side dishes, and yes, as an appetizer (meat wrapped around the apple slice).
An apple by itself is more of an item you can use in between meals to dull that hungry feeling. An apple may become an appetizer by sprinkling with sweetener or sugar and small amount of cinnamon, or a half slice of cheese on a sliced apple pop in the microwave for a minute or two on high.
參考: Myself snacks I make for myself being that I am an diabetic.