Definition of Carbohydrate. Carbohydrate: One of the three nutrient compounds, along with fat and protein, used as energy sources (calories) by the body. Carbohydrates take the form of simple sugars or of more complex forms, such as starches and fiber. Complex carbohydrates come naturally from plants. Intake of complex carbohydrates, when they are substituted for saturated fat, can lower blood cholesterol. Carbohydrates produce 4 calories of energy per gram.