What is Glycosidic bond, disaccharide?

A glycosidic bond joins the anomeric hydroxyl of one sugar to a hydroxyl of another (or of an alcohol, an amine, a base) by condensation, fixing the anomeric carbon in its $\alpha$ or $\beta$ configuration; it is named by the configuration and the carbons joined: $\alpha(1{\to}4)$, $\beta(1{\to}4)$, $\alpha(1{\to}6)$. Two sugars so joined form a disaccharide: maltose (glucose $\alpha(1{\to}4)$ glucose, from starch digestion), lactose (galactose $\beta(1{\to}4)$ glucose, the sugar of milk), sucrose (glucose $\alpha(1{\to}2)\beta$ fructose, the sugar of plant sap and of the kitchen). In sucrose both anomeric carbons are engaged: it is non-reducing and does not open, which is why plants transport it (Chapter 24).