Reaction

A Reaction is a special action taken in response to a trigger defined in the Reaction's description. You can take a Reaction on another creature's turn, and if you take it on your turn, you can do so even if you also take an action, a Bonus Action, or both. Once you take a Reaction, you can't take another one until the start of your next turn. The Opportunity Attack is a Reaction available to all creatures. See also "Opportunity Attacks" and chapter 1 ("Actions").