Plain English Meaning
After one engine quits on a twin, you don't fly with the wings perfectly level. You bank a tiny bit toward the working engine and use rudder to keep the airplane slicing cleanly through the air. Done right, the airplane isn't slipping sideways at all, and that gives you the best chance of climbing.
Full Definition
A multiengine flight condition, following the failure of one engine, in which the airplane is flown with no lateral airflow across the fuselage. It is achieved by establishing a small bank angle of approximately 2 to 3 degrees toward the operating engine, combined with sufficient rudder to keep the ball of the inclinometer slightly deflected toward the operating engine. This configuration produces the lowest and the best .