A coastline is the line where land meets the ocean or sea. If you've ever stood on a beach and looked at where the waves touch the sand, you were standing right at the coastline. But coastlines aren't always smooth, sandy beaches: they include rocky cliffs, muddy marshes, pebbled shores, and everything in between.

Coastlines are surprisingly tricky to measure. At high tide, the ocean reaches further inland than at low tide. Zoom in close, and you'll see every tiny bay and inlet adds more distance. A straight line across a bay might be 10 miles, but if you follow every curve and cove along the actual shore, it could be 50 miles or more. This is why different sources give different measurements for a country's coastline: it depends on how closely you're looking and what you're counting.

Coastlines have shaped human history because they're perfect places for fishing, trade, and travel by ship. Many of the world's largest cities, from New York to Tokyo to Sydney, grew up along coastlines. The word can also describe any long boundary between two different things, like the coastline between childhood and the teenage years, though this usage is less common.