Charging (5e Variant Rule)
From D&D Wiki
A weapon with the Charging property has a type of die in parenthesis after Charging. You can use an action to charge the weapon, increasing its next attack's damage by the die listed after the property. If you become incapacitated, prone, restrained, or your weapon is released by any other means, all the charged energy is depleted with no effect. You can charge a weapon for as many rounds as you want.