Illustrated Glossary of Organic Chemistry

[3+2]: A cycloaddition reaction in which a three-atom component combines with a two-atom component to form a five-membered ring. May be intermolecular or intramolecular.

This intermolecular [3+2] cycloaddition of an oxyallyl cation (the three atom component; shown in red) with an alkyne (the two atom component; shown in blue) produces a new cyclopentane ring.