Fat:1: Along with proteins and carbohydrates, one of the three nutrients used as energy sources by the body. The energy produced by fats is 9 calories per gram. Proteins and carbohydrates each provide 4 calories per gram. 2: Total fat; the sum of saturated, monounsaturated and polyunsaturated fats. Intake of monounsaturated and polyunsaturated fats can help reduce blood cholesterol when substituted for saturated fats in the diet. 3: A slang term for obese or adipose. 4: In chemistry, a compound formed from chemicals called fatty acids. These fats are greasy, solid materials found in animal tissues and in some plants.