Plain English Meaning
Climbing and turning at the same time. The airplane goes up and changes direction in one combined maneuver, but it climbs more slowly than it would in a straight climb because some of the lift is being used to turn.
Full Definition
A maneuver in which the airplane gains altitude while simultaneously banking to change heading. Because the lift vector is tilted away from vertical during the bank, the airplane has less vertical lift available to climb, so the rate of climb is reduced compared to a straight climb at the same power and airspeed. The combination of high pitch attitude, reduced airspeed, and bank increases the load factor on the inside wing and raises the of a , particularly if the turn is .