What is "monofin"

n. A single fin for both foot, worn in finswimming.

A monofin is a type of swimfin typically used in underwater sports such as finswimming, free-diving and underwater orienteering. It consists of a single or linked surfaces attached to both of the diver's feet. The diver's appearance is then reminiscent of a mermaid or merman.

The arrival of the monofin in the early 1970s led to the breaking of all world records by the end of the decade due to the improved performance possible when used in lieu of bi-fins.