Illustrated Glossary of Organic Chemistry

[4+2]: A cycloaddition reaction in which four atoms of one molecule combine with two atoms of another molecule to form a new six-membered ring. May also be intramolecular.

The Diels-Alder reaction between 1,3-butadiene and maleic anhydride is a [4+2] reaction. 1,3-Butadiene contributes four carbons (shown in red) to the new ring, and is the diene. Maleic anhydride contributes two carbons (shown in blue) to the new ring and is the dienophile.