Unity is being together or at one with someone or something. It's the opposite of being divided.
This is a word for togetherness or oneness. When the north won the Civil War, it assured the unity of the United States. Sports teams wear uniforms to show unity, and their fans wear team colors for the same reason. When a bunch of people act as one and are on the same page, they're displaying unity. When people are bickering and disorganized, there's no unity. In any group or cause, unity can be hard to find and maintain.