In retrosynthetic analysis, a synthon is a destructural unit within a molecule which is related to a possible synthetic operation. The term was coined in 1968 by E.J. Corey. He noted in 1988 that the "word synthon has now come to be used to mean synthetic building block rather than retrosynthetic fragmentation structures". It was noted in 1998 that the phrase did not feature very prominently in Corey's 1981 book, The Logic of Chemical Synthesis, as it was not included in the index.