The term GLIDER'S LACK is referring to the lack of a MOTOR and ENGINE for a glider. A MOTOR is a device that converts energy into mechanical motion, while an ENGINE is a machine that converts one form of energy into mechanical energy. Gliders are aircrafts that are propelled by air currents and do not have any type of MOTOR or ENGINE. Therefore, the term GLIDER'S LACK is referring to the fact that gliders do not have a MOTOR or ENGINE.