(1) Technically, the line that forms the boundary between the coast and the shore (2) Commonly, the line that forms the boundary between land and the water (3) The line where terrestrial processes give way to marine processes, tidal currents, wind waves, etc
A country's coastline is the outline of its coast. Thousands of volunteers gave up part of their weekend to clean up the California coastline. the land on the edge of the coast, especially the shape of this land as seen from the air