The sugar of milk is called

The sugar in milk is called 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). The name comes from lac (gen. lactis), the Latin word for milk, plus the suffix -ose used to name sugars.
The compound is a white, water-soluble, non-hygroscopic solid with a mildly sweet taste. It is used in the food industry.