Foil is how areas of thickness correlate with other areas of thickness from nose to tail, deck to bottom and vice versa. Essentially, good foil is an even distribution of thickness as the nose fades into the wide-point and then into the tail. Imagine trying to turn a board to go down the line that has a brick attached to the nose or the tail—the “swing” will be off balance. Good foil translates into good flow as the board moves from points on the nose to the middle and to the tail throughout the duration of a ride.
Foil can also be used to describe some fin designs. Foil in the fin holds the same concept as it does on a surfboard. The thickness must be balanced throughout or the fin will not do what it is made to do—it will be too stiff or too flexy, or it won’t channel water smoothly.