A synonym for @P04491@. The number of 'centres' is the number of atoms not bonded initially, between which single bonds are breaking or new bonds are formed in the @T06468@. This number does not necessarily correspond to the ring size of the @T06468@ for the @P04491@. Thus, a Diels–Alder reaction is a 'four-centre reaction'. This terminology has largely been superseded by the more detailed one developed for the various pericyclic reactions.