Lactose : What is it?

Lactose is a disaccharide sugar derived from galactose and glucose that is found in milk. Lactose makes up around 0-8% of milk (by weight), although the amount varies among species and individuals. It is extracted from sweet or sour whey. The name comes from lac or lactis, the Latin word for milk, plus the -ose ending used to name sugars. It has a formula of C₁₂H₂₂O₁₁.

Info source: http://en.wikipedia.org/wiki/Lactose