What is the principle carbohydrate of milk?

Lactose is the major carbohydrate in the milk of most species.

Lactose is a disaccharide composed of the monosaccharides D-glucose and D-galactose, joined in a ß-1,4-glycosidic linkage. The chemical name for lactose is 4-0-ß-D-galactopyranosyl-D-glucopyranose.