Diels-Alder (type) Cycloaddition
The Diels-Alder reaction involves a diene adding to a dieneophile (an alkene) to give a cycloalkene (or related) product. The Diels-Alder reactions is a 4 + 2 pericyclic process.

For more information look in the Chemogenesis webbook sections on addition reactions, Lewis acid/base interaction matrix. and pericyclic reaction chemistry.