A semi-automatic shotgun is a shotgun that is able to fire a shell after every trigger pull, without needing to manually chamber another round. These weapons use gas, blowback, or recoil operation to cycle the action, eject the empty shell, and load another round.
Many semi-automatic shotguns also provide an optional manual means of operation such as by pump action or a charging handle.
References
Semi-automatic shotgun Wikipedia(Text) CC BY-SA