What is delta?

n.
The fourth letter of the Greek alphabet.
An object shaped like a triangle.
A usually triangular alluvial deposit at the mouth of a river.
A similar deposit at the mouth of a tidal inlet, caused by tidal currents.
Mathematics. A finite increment in a variable.

Low-lying plain composed of stream-borne sediments deposited by a river at its mouth. Deltas have been important to humankind since prehistoric times. Sands, silts, and clays deposited by floodwaters were extremely productive agriculturally; and major civilizations flourished in the deltaic plains of the Nile and Tigris-Euphrates rivers. In recent years geologists have discovered that much of the world's petroleum resources are found in ancient deltaic rocks. Deltas vary widely in size, structure, composition, and origin, though many are triangular (the shape of the Greek letter delta).

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