A melee attack made with disadvantage against an adjacent enemy as it willingly moves away. Monsters do not make opportunity attacks, only heroes can.
Opportunity Attack
Core Rules v2 p. 14
Related Rules
6Advantage & DisadvantageFundamentals / Stats, Skills & Checks
AttackCombat / Actions
CoverCombat / Positioning & Control
Heroic ReactionsCombat / Reactions
MoveCombat / Actions
MovementAdventuring / Movement, Range & Space