Delta (/ˈdɛltə/;^[1] uppercase Δ, lowercase δ; Greek: δέλτα, délta, [ˈðelta])^[2] 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 𐤃.^[3] Letters that come from delta include Latin D and Cyrillic Д.

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A river delta (originally, the delta of the Nile River) is so named because its shape approximates the triangular uppercase letter delta. Contrary to a popular legend, this use of the word delta was not coined by Herodotus.^[4]

In Ancient Greek, delta represented a voiced dental plosive IPA: [d]. In Modern Greek, it represents a voiced dental fricative IPA: [ð], like the "th" in "that" or "this" (while IPA: [d] in foreign words is instead commonly transcribed as ντ). Delta is romanized as d or dh.

The uppercase letter Δ is used to denote:

• Change of any changeable quantity, in mathematics and the sciences (in particular, the difference operator^[5][6]); for example, in
  $${\displaystyle {\frac {y_{2}-y_{1}}{x_{2}-x_{1}}}={\frac {\Delta y}{\Delta x}},}$$

  
  the average change of y per unit x (i.e. the change of y over the change of x). Delta is the initial letter of the Greek word διαφορά diaphorá, "difference". (The small Latin letter d is used in much the same way for the notation of derivatives and differentials, which also describe change by infinitesimal amounts.)
• The Laplace operator:

  ${\displaystyle \Delta f=\sum _{i=1}^{n}{\frac {\partial ^{2}f}{\partial x_{i}^{2}}}.}$

• The discriminant of a polynomial equation, especially the quadratic equation:^[7][8]

  ${\displaystyle \Delta =b^{2}-4ac.}$

• The area of a triangle

  ${\displaystyle \Delta ={\tfrac {1}{2}}ab\sin {C}.}$

• The symmetric difference of two sets.
• A macroscopic change in the value of a variable in mathematics or science.
• Uncertainty in a physical variable as seen in the uncertainty principle.
• An interval of possible values for a given quantity.
• Any of the delta particles in particle physics.
• The determinant of the matrix of coefficients of a set of linear equations (see Cramer's rule).
• 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.
• A simplex, simplicial complex, or convex hull.
• In chemistry, the addition of heat in a reaction.
• In legal shorthand, it represents a defendant.^[9]
• In the financial markets, one of the Greeks, describing the rate of change of an option price for a given change in the underlying benchmark.
• A major seventh chord in jazz music notation.
• In genetics, it can stand for a gene deletion (e.g. the CCR5-Δ32, a 32 nucleotide/bp deletion within CCR5).
• The American Dental Association cites it (together with omicron for "odont") as the symbol of dentistry.^[10]
• The anonymous signature of James David Forbes.^[11]
• Determinacy (having a definite truth-value) in philosophical logic.
• In mathematics, the symbol ≜ (delta over equals) is occasionally used to define a new variable or function.^[12]

The lowercase letter δ (or 𝛿) can be used to denote:

• A change in the value of a variable in calculus
• A functional derivative in functional calculus
• An auxiliary function in calculus, used to rigorously define the limit or continuity of a given function
• The Kronecker delta in mathematics
• The degree of a vertex (graph theory)
• The Dirac delta function in mathematics
• The transition function in automata
• Deflection in engineering mechanics
• The force of interest in actuarial science
• The chemical shift of nuclear magnetic resonance in chemistry
• The relative electronegativity of different atoms in a molecule, δ⁻ being more electronegative than δ⁺
• Text requiring deletion in proofreading; the usage is said to date back to classical times
• In some of the manuscripts written by Dr. John Dee, the character of delta is used to represent Dee
• A subunit of the F1 sector of the F-ATPase
• The declination of an object in the equatorial coordinate system of astronomy
• The dividend yield in the Black–Scholes option pricing formula
• Ratios of environmental isotopes, such as ¹⁸O/¹⁶O and D/¹H from water are displayed using delta notation – δ¹⁸O and δD, respectively
• The rate of depreciation of the aggregate capital stock of an economy in an exogenous growth model in macroeconomics^[13]
• In a system that exhibits electrical reactance, the angle between voltage and current
• Partial charge in chemistry
• The maximum birrefringence of a crystal in optical mineralogy.^[14]
• An Old Irish voiced dental or alveolar fricative of uncertain articulation, the ancestor of the sound represented by Modern Irish dh

• Greek Delta / Coptic Dalda

• Latin Delta

• Technical and Mathematical symbols

• Mathematical Delta

These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

Look up Δ or δ in Wiktionary, the free dictionary.

• Arrow (symbol)
• Chevron (insignia)
• ∆ (disambiguation)
• D, d
• Д, д
• ẟ - Latin delta
• ∂ - the partial derivative symbol, sometimes mistaken for a lowercase Greek letter Delta.
• ð - the small eth appears similar to a small delta, and also represents a d sound in some contexts
• Th (digraph)
• Thorn (letter)
• Greek letters used in mathematics, science, and engineering
• ∇ - Nabla symbol
• Delta Air Lines
• SARS-CoV-2 Delta variant