The two terms are used interchangeably in advertising and describe genuinely different mechanisms. The distinction comes down to whether the system permits the front and rear axles to turn at different speeds.
Turning forces the axles to differ
When a vehicle corners, the front wheels follow a larger arc than the rear wheels and therefore travel farther in the same time.
A drivetrain that rigidly connects both axles cannot accommodate that difference. Something has to give, and on grippy pavement the driveline winds up under stress.
That condition, called driveline binding, produces a shuddering feel in tight turns and loads the transfer case, universal joints and axle shafts.
Part-time four wheel drive accepts the tradeoff
Traditional four wheel drive locks the front and rear outputs of the transfer case together, guaranteeing that both axles receive torque regardless of which wheels are slipping.
On loose surfaces the tires slip slightly and relieve the speed difference, so binding does not occur. That is why the system is intended for dirt, snow and gravel.
The owner selects it and must deselect it before returning to dry pavement, which is the practical reason the system is called part-time.
All wheel drive adds a differential in the middle
All wheel drive places a center differential, viscous coupling or clutch pack between the axles, allowing them to rotate at different speeds continuously.
That makes the system usable on any surface at any time, with no driver action required, which suits vehicles designed primarily for pavement.
The cost is that an open center differential sends torque along the path of least resistance, so a single spinning wheel can stop the vehicle moving without additional control.
Electronics filled the gap
Modern systems use the brakes to slow a spinning wheel, which forces torque across the differential to the wheel with grip, imitating a locker without one.
Clutch-based couplings go further, varying how much torque reaches the secondary axle based on wheel speed, throttle position and steering angle.
These strategies work well within their limits and depend on brake and clutch heat capacity, which is why sustained low-speed climbing still favors mechanical locking.
Low range is the other real difference
Serious four wheel drive systems include a reduction gearset providing a much lower overall ratio, multiplying torque and slowing the vehicle for controlled descent.
Most all wheel drive crossovers have no low range, since the transmission and driveline are not built for the loads that gearing produces.
Tires and ground clearance then decide the rest. Neither system creates traction, it only distributes what the tires can already generate.