Delta (upside down A, uppercase Δ, lowercase δ) is the fourth letter of the Greek alphabet. In the system of Greek numerals it has a value of 4. It was derived from the Phoenician letter Dalet . Letters that arose from Delta include the Latin D and the equivalent in the Cyrillic alphabet Д.
In Modern Greek it represents a voiced dental fricative /ð/, (like the th in the English word this) but in the Ancient Greek language, it represented a voiced dental plosive [d].
A river delta is named after the letter delta because it has roughly the triangular shape of the upper-case delta.
The upper-case letter Δ can be used to denote:
= the average change of y per unit x commonly known as the change of y over the change of x.


The lower-case letter δ can be used to denote:

The difference operator, effecting a change or difference between mathematical values:
By extension of the above, change generally, a use which shows up frequently in medical charts.
The Laplace operator:
The discriminant of the quadratic equation:
A macroscopic change in the value of a variable in mathematics or science.
Any of the delta particles in particle physics.
That an associated locant number represents the location of a covalent bond in an organic compound, the position of which is variant between isomeric forms.
In legal shorthand, it represents a defendant.
In genetics, it can stand for a gene deletion, e.g. the CCR5-Δ32 a deletion of the CCR5 at the 32nd bp segment.
In medical shorthand, it can stand for change of any type.
An infinitesimal change in the value of a variable in mathematics or science.
An auxiliary function in Calculus used to rigorously define the limit or continuity of a given function.
The Kronecker delta in mathematics.
The Dirac delta function in mathematics.
Deflection in engineering mechanics
Text requiring deletion in proofreading. The usage is said to date back to classical times.
The relative electronegativity of different atoms in a molecule, δ