The natural phenomenon that forms a boundary between land and sea is called the coastline. Coastlines are dynamic interfaces where terrestrial ecosystems meet aquatic environments, resulting in unique habitats that are rich in biodiversity. They are influenced by various factors, including tides, waves, and currents, which can sculpt the landscape over time. Coastal zones are also vital for human activities, including fishing, tourism, and transportation, and are often subjected to challenges from climate change and pollution.