The chemical reactions and pathways resulting in the breakdown of a monosaccharide (generally glucose) into pyruvate, with the concomitant production of a small amount of ATP. Glycolysis begins with phosphorylation of a monosaccharide (generally glucose) on the sixth carbon by a hexokinase, and ends with the production of pyruvate. Pyruvate may be converted to ethanol, lactate, or other small molecules, or fed into the TCA cycle.