Translational lift (also known as Effective Translational Lift, or ETL) is a transitional state present after a helicopter has moved from hover to forward flight. This state provides extra lift, most typically, when the airspeed reaches approximately 16-24 knots, but is present with any horizontal flow of air across the rotor, and therefore can be present without any forward motion of the aircraft, given prevailing wind conditions. As a result, the tail rotor also becomes more efficient due to the wind bubble that is formed around the helicopter from progressively less turbulent air.
The most common example of the effect is this: a helicopter might be slightly overloaded for take off so that it cannot hover in ground effect. Liftoff can still be achieved if the helicopter has enough of a straight runway to make a "running take off" where the pilot will slowly accelerate the helicopter across the ground on its landing gear until translational lift speed is achieved, extra rotor-disc lift is produced, and the aircraft will begin to climb.
In Robert Mason’s book Chickenhawk, Mason describes a situation where an aircraft, loaded with 2500 pounds of high explosives, was above gross weight and should have been unable to achieve flight, but was actually "coaxed" into flight via the "running take off".