Definition: Foil refers to the overall flow and balance of a surfboard's final shape. This includes how the nose section meets the midpoint to the tail. To see the foil of a board, hold a board by the nose with the tail on the carpet and look down the rail and bottom of the board. See that flow? Now run your hand down the rail. The board should be smooth and balanced throughout the shape. That is the foil, the overall distribution of foam.