Ground swell

Technically, this is a swell which, on reaching a depth of less than half its wave-length, starts to become shorter and steeper, preparatory to breaking in even shallower water. In that sense it is a wave-train which is influenced by the nearness of the ground. But to most yacht skippers, a ground swell is just a swell which arrives from far off when there is no local wind to set up seas of the same height.