A "single fin" refers to a type of surfboard design that features one large fin at the tail. This design is commonly associated with traditional longboards and some retro shortboards. The single fin provides stability and allows for smooth, flowing turns, making it ideal for cruising on smaller waves. Single fin surfboards are often favored by surfers who enjoy a more relaxed style of riding. They can enhance the board's performance in certain conditions, allowing for a more graceful and controlled experience on the water. This design contrasts with multi-fin setups, which offer different handling characteristics.