@[email protected] (usually for @[email protected] transfer) by a bifunctional @[email protected] involving a mechanism in which both @[email protected] are implicated in the @[email protected], so that the corresponding @[email protected] is larger than that expected for @[email protected] by chemical species containing only one of these functional groups. The term should not be used to describe the @[email protected] action of two different catalysts ('concerted @[email protected]').