The delta of a river is the flat, often triangular area of land where the river splits into smaller channels as it flows into the ocean or a lake. As a river nears the end of its journey, it slows down and drops the sediment (sand, silt, and soil) it's been carrying. Over thousands of years, this sediment builds up into new land, creating a delta. The Nile Delta in Egypt and the Mississippi Delta in Louisiana are famous examples.

If you looked at satellite images of where the Nile meets the Mediterranean Sea, you'd see how the river fans out into multiple channels, creating that distinctive delta shape.

In math and science, delta (represented by the symbol Δ) means change or difference. When scientists talk about “delta T,” they mean the change in temperature. If the temperature rises from 65°F to 72°F, the delta T is 7 degrees. Pilots and engineers use delta to describe how much something has changed between two measurements.