The addition of a compound with a double bond having an allylic hydrogen (the 'ene') to a compound with a multiple bond (the 'enophile') with transfer of the allylic hydrogen and a concomitant reorganization of the bonding, as illustrated below for propene (the 'ene') and ethene (the 'enophile'). The reverse is a 'retro-ene' reaction.
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Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1112 (https://doi.org/10.1351/pac199466051077)