Heir presumptive

Heir presumptive is somebody who is likely to become the king or queen, but there is no guarantee that they will become the ruler. For example, if a king or queen has a son, the son would be the heir presumptive. If something happens to the king or queen, then the son would be next in line to become the ruler. But if something happens to the son, then the next person in line would become the heir presumptive.