The second to last product in glycolysis. Its IUPAC name would be 2-phospho-2-propenoic acid. It is called PEP for short, and this is probably the most appropriate acronym I have seen in biochemistry because it contains a very high energy bond. Its conversion to pyruvate (the final product of glycolysis) by the enzyme pyruvate kinase is accompanied by such a large release of energy (which is stored in ATP) that is is functionally irreversible.