Stunning Strike is a Monk feature gained at level 5 in D&D 5e. Starting at 5th level, you can interfere with the flow of ki in an opponent's body.
Starting at 5th level, you can interfere with the flow of ki in an opponent's body. When you hit another creature with a melee weapon attack, you can spend 1 ki point to attempt a stunning strike. The target must succeed on a Constitution saving throw or be stunned until the end of your next turn.