A reversal is a change of decision or direction, often to the opposite. You liked history, but thought you'd major in business because there were more jobs. Then you took an economics class and hated it. So in a reversal, you majored in history after all.
In legal terms, a reversal is when a high court decides the decision of a lower court is incorrect and should be overturned. In a reversal, the Supreme Court overturned the guilty verdict of the lower court, and the prisoner was set free. A setback or a change in circumstances for the worse is also a reversal. If you make poor investments, you may suffer a reversal in fortune and go from shopping for designer clothes to shopping at thrift stores.