Reaction

A Reaction is a special action you take instantly in response to a trigger, which can occur on your turn or on another creature's turn.

Trigger required: A Reaction only happens in response to a defined trigger granted by a spell, feature, or situation. The most common is the Opportunity Attack (made when a creature you can see leaves your reach).
Timing: The Reaction resolves immediately after its trigger unless its description says otherwise. If it interrupts another creature's turn, that creature finishes its turn once the Reaction ends.
One per round: After taking a Reaction, you can't take another until the start of your next turn.
Independent of action/bonus action: Spending your Reaction doesn't use your action or Bonus Action.
Readied actions: Using the Ready action lets you spend your Reaction to act on a trigger you defined.