A
barrier against
erosion from
longshore currents, constructed at a
right angle to the
shore.
Since longshore currents move sand and soil across the shore, a series of groynes projecting from the land into the water can slow lateral erosion in the area.
However, as one might expect, this tends to affect the erosion rates in adjacent areas -- so the neighbors will have to keep up with the Joneses by building groynes of their own, if they happen to be able to afford it.
(Gleaned from Rand McNally Atlas of the Oceans and paraphrased in a clean room implementation.)