Final answer:
Disaccharides are formed when two monosaccharides undergo a dehydration reaction, combining the hydroxyl group of one monosaccharide with the hydrogen of another, releasing water and forming a covalent bond. This bond is called a glycosidic bond and can be alpha or beta.
Explanation:
Disaccharides (di- = "two") form when two monosaccharides undergo a dehydration reaction (also known as a condensation reaction or dehydration synthesis). During this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another monosaccharide, releasing a molecule of water and forming a covalent bond.
A covalent bond formed between a carbohydrate molecule and another molecule (in this case, between two monosaccharides) is known as a glycosidic bond or glycosidic linkage. Glycosidic bonds can be of the alpha or beta types.