Question
The Diels Alder reaction is a concerted reaction. Define concerted.
Answer
Concerted means that the bond changes occur in a single elementary step, without a discrete reaction intermediate.
In the usual Diels–Alder cycloaddition, a conjugated diene and a dienophile reorganize six π electrons through one cyclic transition state. Two new carbon–carbon σ bonds and one new π bond form as the original π bonds reorganize. There is no separate carbocation or radical intermediate between the reactants and cycloadduct in this mechanism.
Concerted does not require perfectly synchronous bond formation
The two new σ bonds need not develop to exactly the same extent at the transition state. A reaction can be asynchronous yet concerted: one bond is further along, but no intermediate separates two steps.
For this definition question, the decisive distinction is one step with no intermediate, rather than several steps with an intermediate between them.
Evidence boundary
This answer concerns the conventional concerted Diels–Alder mechanism requested by the prompt. Concerted does not mean that both new bonds must form equally far at every instant; unusual substrates or conditions require separate mechanistic evidence.
Sources
These references support the concepts and methods used in the explanation above.