I came across this word on a particular type of hydraulic motor, but I do not understand what is so "orbital" about orbital motor. Essentially, to my understanding, it is just a motor powered by hydraulics and the shaft rotate in high torque. I do not see anything moves in orbital shape... Is Orbital Motor is just a synonyms for hydraulic motor?
ANSWER No Orbital motor and hydraulic motor are not synonymous terms.
A hydraulic motor is essentially a hydraulic pump in reverse. It has one or more pistons that move in a cylinder, with valves and timing, rather like an internal combustion engine powered by compressed air.
By comparison, here's an orbital motor.
Main advantages are that the "motor" itself is quite small, and generates little heat compared to an electric motor. Also, this is a high torque but low speed shaft, so there's no need for a gearbox in situations where an electric motor would need a gearbox.
Downsides, its low speed, so if you want to move something quickly, it may not be the best motor for the job. You'll also need pressurised lines to and from the motor to your pump.
Here's a cutaway showing more of how it works. Notice the shaft in the middle is not flat - this is the rotor shaft and is seen in the middle of the blue picture above, and is not flat.