[4 +2] Cycloaddition

Definition

A pericyclic reaction, also called a Diels-Alder reaction, that takes place between two different pi systems, one
of which is associated with four atoms while the other is associated with two atoms.

Explanation

In a Diels-Alder reaction, there are two different π systems involved. The first involves 4 atoms while the second involves two atoms, as shown in the diagram below:

A basic reaction scheme of a Diels-Alder reaction

As you can see, the dienophile attacks the diene (the substituted alkene seeking a diene to react with) in a concerted process (meaning it happens all at the same time), forming a cyclohexene.