In the Gospel according to Luke, someone asks Jesus that question and he replies with the story of a man who was beaten and robbed and left on the side of the road.
One man who was busy passed by, saw the man, but hurried on. Another man passed by but didn’t want to help him either. The third man who came along stopped to help. He bound up the man’s wounds, put him on his own donkey and took him to an inn where he paid the innkeeper to care for him.
So who was the neighbor? The one who came alongside, the one who cared for someone else, he was the neighbor.