Four-Quadrant Drive
It is the form of drive in which the motor can be both driven and braked in either direction. It covers all four regions of the speed–torque plane; it requires a full H-bridge and independent control of four switches. It is the structure sought in lifting applications and those working against gravity.
Four quadrants cover these four cases: forward driving, forward braking, reverse driving, reverse braking. In the braking regions the torque reverses against the direction of motion; the motor behaves like a generator and energy passes from the mechanics to the electrics.
The typical situation where this structure is necessary is when the load tries to turn the motor. While a load is being lowered, gravity accelerates the motor; a controlled descent is only possible with a driver that can work in the braking region. Otherwise the load accelerates with something close to free-fall acceleration.
Two points are watched in practice. First, the energy released during braking has to go somewhere: a battery supply can absorb it, a switch-mode power supply cannot and the line voltage climbs. Second, electronic braking can stop the load but cannot hold it in the air indefinitely; on loads that must remain suspended, a spring-locking mechanical brake is mandatory when power is removed.
The context in which this term is used: DC Motor Driver Selection Guide