Definitions:
the fourth letter of the greek alphabet (δ, δ), transliterated as ‘d’.

a code word representing the letter d, used in radio communication.

a difference between two things or values

variation of a variable or function.

a finite increment.

the discriminant of a quadratic equation.

declination.

a triangular tract of sediment deposited at the mouth of a river, typically where it diverges into several outlets