Any of several water-soluble simple carbohydrates. The principal categories of sugars are monosaccharides, disaccharides, and polysaccharides. A monosaccharide is a single sugar, such as glucose, fructose, or galactose. A disaccharide is a double sugar, such as sucrose (table sugar) or lactose. A polysaccharide is a sugar made up of repeating units of glucose, such as cellulose, starch, and glycogen. See also carbohydrate, fructose, galactose, glucose, saccharide, sucrose.