Appetite is wanting to eat food, the feeling of hunger. Humans and animals have appetite to make sure that they eat enough food to survive. A loss of appetite is also called anorexia (this is not the same as the eating disorder anorexia nervosa).
Appetite can change when people are ill, when they have eating disorders and when they are on different types of medications. A big appetite can make somebody eat more than they need to, and they can put on weight, even becoming obese. A small appetite can make someone eat less than they need to, making them lose weight or become malnourished.