a person who unselfishly helps another in a time of need
—When I was traveling, my phone ran out of power, but a good Samaritan let me use her phone to make a call.

One day, an expert in Jewish law tested Jesus by asking what a person must do to gain eternal life. Jesus asked him what the Old Testament said, and the man answered that a person should love God (which comes from a verse in Deuteronomy) and should love his neighbor (which is from Leviticus):

You must not take vengeance or bear a grudge against the children of your people, but you must love your neighbor as yourself [love thy neighbor as thyself]. I am the Lord. (Leviticus 19:18)

At another time, Jesus told a Jewish teacher the same thing, saying,

The most important is: “Listen, Israel, the Lord our God, the Lord is one. Love the Lord your God with all your heart, with all your soul, with all your mind, and with all your strength.” The second is: “Love your neighbor as yourself [love thy neighbor as thyself].” There is no other commandment greater than these. (Mark 12:29-31)

After Jesus told the expert in the law that his answer was correct, the man asked who his neighbor was. Jesus then told this parable:

“A man was going down from Jerusalem to Jericho, and fell into the hands of robbers, who stripped him, beat him up, and went off, leaving him half dead. Now by chance a priest was going down that road, but when he saw the injured man he passed by on the other side. So too a Levite, when he came up to the place and saw him, passed by on the other side. But a Samaritan who was traveling came to where the injured man was, and when he saw him, he felt compassion for him. He went up to him and bandaged his wounds, pouring oil and wine on them. Then he put him on his own animal, brought him to an inn, and took care of him. The next day he took out two silver coins and gave them to the innkeeper, saying, ‘Take care of him, and whatever else you spend, I will repay you when I come back this way.’ Which of these three do you think became a neighbor to the man who fell into the hands of the robbers?” The expert in religious law said, “The one who showed mercy to him.” So Jesus said to him, “Go and do the same.” (Luke 10:30-37)

Jesus was telling the man that all people were his neighbors, and, therefore, he should love everyone. In his story, the man from Samaria showed this by helping a Jew, even though the Samaritans and Jews normally treated each other as enemies. 

Today, love thy neighbor, or love your neighbor, is often used to mean simply “be kind to others.” While a person who shows kindness to other people is called a good Samaritan, or a Samaritan, a person who ignores someone in need is sometimes said to “pass by on the other side” (“walk by on the other side“).