What is the difference of crocodile and aligator?

Crocodile : Any of several large carnivorous thick-skinned long-bodied aquatic reptiles (family Crocodylidae) of tropical and subtropical waters
嘴長、條尾扁

Alligator: (Alligator mississippiensis of the southeastern United States and A. sinensis of China) Having broad heads not tapering to the snout and a special pocket in the upper jaw for reception of the enlarged lower fourth tooth
(產於美國及中國的)短吻鱷