Illustrated Glossary of Organic Chemistry

[4+3]: A cycloaddition reaction in which a four-atom component combines with a three-atom component to form a new seven-membered ring. The reaction may be intermolecular or intramolecular.

In this intermolecular [4+3] cycloaddition reaction, furan (the four-atom component; shown in red) combines with an oxyallyl cation (the three-atom component; shown in blue) to form a new cycloheptane ring.