Most commonly, a thonged sandal with a rubber sole. Some flip-flops have a single band across the toes, some have loops for all the toes to go through, etc. There are different designs that are all part of the same family because of the distinctive sound they make. If the sandal has any kind of a back, it is most likely not a flip-flop, as a back of any kind inhibits the shoe's ability to flip and flop. It's always a good idea to wear flip-flops in locker rooms, public showers, and any other place people are likely to have trod their sweaty feet across.