Final answer:
Disaccharides form when two monosaccharides undergo a dehydration reaction and are linked together by a glycosidic bond.
Step-by-step explanation:
Disaccharides 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. This covalent bond is called a glycosidic bond or glycosidic linkage, which can be of the alpha or beta type.