Answer: A concerted reaction is one in which all bond-making and bond-breaking events occur in a single elementary step via a single transition state, with no discrete intermediates.
Explanation: In the Diels–Alder reaction this means the two new σ bonds form and the π bonds reorganize simultaneously in a cyclic (pericyclic) transition state. Concertedness explains the reaction’s stereospecificity and the absence of ionic or radical intermediates. (Note: “concerted” does not require perfectly equal timing — the process can be asynchronous yet still concerted because no intermediate is formed.)