Milk sugar is

Milk sugar is Lactose.
Lactose is a disaccharide sugar synthesized by galactose and glucose subunits and has the molecular formula C₁₂H₂₂O₁₁. Lactose makes up around 2 - 8% of milk (by mass).
Lactose is a disaccharide derived from the condensation of galactose and glucose, which form a β-1→4 glycosidic linkage.
The lactose content of dairy products can vary depending on the type of product and the specific brand or variety.