Disaccharide

A disaccharide is a carbohydrate polymer composed of two sugar monomers (monosaccharides) which are joined by a glycosidic bond formed by a condensation reaction. Disaccharides are the simplest forms of polysaccharides. 

Examples of disaccharides:

Maltose: two monomers of alpha-glucose joined by an alpha 1,4 glycosidic bond. 

Lactose: one galactose monomer and one glucose molecule joined by a beta 1,4 glycosidic bond. Lactose is the major sugar in milk.

Sucrose: one glucose monomer and one fructose monomer joined by an alpha 1,2 glycosidic bond. Sucrose is a product of photosynthesis in plants.

References:

Molecular Cell Biology 2013 (Seventh Edition) Lodish et al page 39