money and material possessions, pictured in Jesus' teaching as a rival master that competes with God for a person's trust and service.
Mammon began as an ordinary, everyday word. In the language Jesus spoke, it simply meant the property a person owns: money, goods, whatever one has. Jewish teachers of the time used it with no hint of evil, telling people to guard a neighbor's money as carefully as their own, and even to honor God with their wealth by giving generously. The word itself was neutral.
What makes it striking in the Gospels is what Jesus does with it. Rather than translate the foreign word, the Gospel writers kept it and set it up as if it were a person, a master. "You cannot serve God and mammon" puts wealth in the place of a rival lord that demands the same whole-life devotion which belongs to God alone. The point is not that money is evil, but that it competes for the loyalty of the heart.
In Luke's Gospel Jesus twice calls it "unrighteous mammon," or "the mammon of unrighteousness." This is not a charge that the money was stolen. It is a way of speaking, common in the Jewish writings of that era, that labels wealth by the corrupt world it belongs to: money as it works in a fallen age, tangled up with injustice and bound to pass away. Against it Jesus sets the "true riches," the lasting wealth of God's kingdom, and asks whether people who cannot be trusted with mere money can be trusted with that.