Delta (upper case Δ, lower case δ) is the 4th letter of the Greek alphabet. In the system of Greek numerals is has a value of 4.
In mathematics and science, delta is used with a variable to indicate a change in the value of that variable. Usually, an upper case delta (e.g. Δx) is used for large or macroscopic changes, and a lower case delta (e.g. δx) is for small or microscopic changes.
The lower-case letter δ is used in proofreading to indicate deletion. The usage is said to date back to classical times.
The term river delta is named after the letter delta because it has roughly the shape of the upper-case delta.