Plain English Meaning
A button you press on the autopilot to climb or descend to a new altitude. Power stays at a chosen setting, and the autopilot raises or lowers the nose to keep the airplane at the speed you want until it levels off at the new altitude.
Full Definition
An autopilot vertical mode that commands a climb or descent to a pilot-selected altitude while the autothrottle (or pilot manually) sets a fixed power setting, with the autopilot pitching the aircraft up or down as needed to hold a target airspeed or Mach number during the change.