Sea level is the average height of the ocean's surface, used as the starting point for measuring how high or low land is. When a mountain is listed as 14,000 feet tall, that means 14,000 feet above sea level. When Death Valley is described as 282 feet below sea level, it means the ground there sits lower than the ocean's surface.

The oceans connect to each other around the world, so their surfaces stay at roughly the same height everywhere (ignoring waves and tides). This makes sea level a reliable reference point. Without it, we'd have no consistent way to compare elevations: is Colorado higher than Switzerland? Sea level gives us the answer.

Sea level isn't perfectly flat or constant. Tides raise and lower it twice daily, which is why scientists use mean sea level (average) measured over many years. The ocean's height also changes slowly over decades as ice melts or freezes at the poles and as water temperatures change.

When pilots talk about altitude, they're measuring height above sea level, not above the ground directly below them. This matters when flying over mountains. A plane at 15,000 feet altitude is safely above a 10,000-foot peak, but dangerously low near a 16,000-foot mountain.