Why is it called a delta?

It is called a delta because of its triangular shape that resembles the Greek letter delta. The term was first used by the ancient Greek historian Herodotus in the 5th century BC to describe the mouth of the Nile River in Egypt. As the Nile approaches the Mediterranean Sea, it splits into many distributaries that deposit sediment and form a large, fertile, fan-shaped plain. From above, this plain looks like the uppercase Greek letter Δ, which is a triangle. Herodotus noticed this similarity and called the region the Delta, after the letter. The name stuck and was later applied to all similar river mouth landforms around the world, such as the Mississippi Delta, Ganges Delta, and Mekong Delta. The Greek letter delta itself comes from the Phoenician letter dalet, meaning door, which also had a triangular shape. In modern usage, delta is still called delta for the same reason. The mathematical symbol Δ is also called delta because it uses the same Greek letter to represent change, and the river delta is called delta because of its shape. So the name is purely visual, based on the resemblance between the landform and the triangular Greek letter.