Fat can refer to the component of food, or is sometimes used as an adjective to describe the physique of a person or animal with a high body mass index. Fat is a term used to describe a variety of organic compunds that help to make up the composition of organisms. In science, the term "fat" is usually referred to as a lipid (also known as a fat cell). There are approximately 45 billion fat cells in an average adult.
People who are excessively fat suffer from obesity. People who are not excessively fat are often referred to as thin. People who have changed their eating habits to lose weight so they can shift from the fat category to the thin category are often said to be on a diet.
Animals in nature rarely get fat, as they are too busy foraging for food to be able to accumulate the amount of fat stores needed to be considered fat. Some animals gain fat deposits as winter approaches to allow themselves to hibernate.