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Greek alphabet
Α α Alpha	Ν ν Nu
Β β Beta	Ξ ξ Xi
Γ γ Gamma	Ο ο Omicron
Δ δ Delta	Π π Pi
Ε ε Epsilon	Ρ ρ Rho
Ζ ζ Zeta	Σ σ ς Sigma
Η η Eta	Τ τ Tau
Θ θ Theta	Υ υ Upsilon
Ι ι Iota	Φ φ Phi
Κ κ Kappa	Χ χ Chi
Λ λ Lambda	Ψ ψ Psi
Μ μ Mu	Ω ω Omega
Obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi
Ϛ ϛ Stigma	Ϸ ϸ Sho

For other uses, see Delta.

Delta (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 difference operator, effecting a change or difference between mathematical values:

<math>{y_2-y_1\over x_2-x_1} = {\Delta y \over \Delta x} </math> = the average change of y per unit x commonly known as the change of y over the change of x.

• By extension of the above, change generally, a use which shows up frequently in medical charts.
• The Laplace operator:

<math>\Delta f = \sum_{i=1}^n {\frac{\partial^2}{\partial x_i^2}} </math>

• The discriminant of the quadratic equation:

<math>\Delta = b^2 - 4ac\,\!</math>

• A macroscopic change in the value of a variable in mathematics or science.
• Any of the delta particles in particle physics.
• The defendant, using legal shorthand.
• 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.

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

• An infinitesmal 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 electronegativity of different atoms in a molecule, δ^- being more electronegative than δ⁺.
• In some of the Manuscripst written by Dr. John Dee, the charecter of delta is used to represent Dee.als:Δ

br:Delta (lizherenn) bg:Делта ca:Delta (lletra) da:Delta (bogstav) de:Delta arc:Δ el:Δέλτα es:Δ eu:Delta (greko) fa:دلتا fr:Delta ga:Deilte gd:Delta gl:Delta (letra) ko:Δ hr:Delta it:Delta (lettera) he:דלתא (אות) la:Delta hu:Delta nl:Delta (letter) ja:Δ no:Delta (bokstav) nn:Delta nds:Delta pl:Delta pt:Δ ru:Дельта (буква) simple:Delta fi:Delta sv:Delta tr:Delta zh:Δ