Delta

Etymology

From the Greek letter delta (Δ), whose triangular shape the historian Herodotus saw mirrored in the sprawling mouth of the Nile. The letter’s name passed to the landform and then to any river mouth of similar form.

noun · /DEL-tuh/
A fan-shaped deposit of sediment where a river splits into channels and empties into a sea or lake.
What it names
The landform a river builds at its mouth — and carries a neat visual etymology worth knowing.
Example
The river fanned into a maze of channels across the delta.
Related idea
A delta builds land outward with sediment; an estuary is drowned by the tide reaching in — the same river mouth can be one or the other depending on tides and sediment.
noun · /DEL-tuh/
The fourth letter of the Greek alphabet (Δ), widely used in mathematics and science to mean a difference or change in a quantity.
What it names
a Greek letter and the symbol for change
Example
The delta between last year’s revenue and this year’s was small.
Related idea
The river delta is named after this triangular letter shape; in science the same symbol marks how much a value has shifted. Compare variance and marginal.