Avoiding Action
An avoiding action is just what it means, it's a turn that needs to be done immediately to avoid the loss of approved seperation between two aircraft. When ATC issues an immediate turn or an avoiding action the controller wants you to make the turn as quickly and SAFELY as the aircraft can.
I've used this phrase to some pilots who appear to take their time in turning, "Start an immediate turn to XXX or become a hood ornament on an Airbus" (or whatever aircraft is trying to run them down). The pilot ususally understands and makes the turn.
Mike R