Consent
Examples
In football, the players have consented to have other players crash into them. If you just tackle someone on the street, that's battery.
Careful about the specific terms of the consent. For example, if you agree to be at work until 5:00 pm, then the boss keeping you there is allowed – but ONLY until 5:00 pm. After that time, you are no longer consenting to be there and must be permitted to leave.
Definition
The person who was harmed gave the acting party permission to take that action.