A transformation involving fusion of a new ring to a molecule via two new bonds. Some authors use the term 'annelation' for the fusion of an additional ring to an already existing one, and 'annulation' for the formation of a ring from one or several acyclic precursors, but this distinction is not made generally.
See also: cyclization
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1083