# Delta (letter): Wikis

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# Encyclopedia

 Greek alphabet Obsolete letters Other characters Αα Alpha Νν Nu Ββ Beta Ξξ Xi Γγ Gamma Οο Omicron Δδ Delta Ππ Pi Εε Epsilon Ρρ Rho Ζζ Zeta Σσς Sigma Ηη Eta Ττ Tau Θθ Theta Υυ Upsilon Ιι Iota Φφ Phi Κκ Kappa Χχ Chi Λλ Lambda Ψψ Psi Μμ Mu Ωω Omega Digamma Qoppa San Sampi Stigma Sho Heta Greek diacritics

Delta (uppercase Δ, lowercase δ; Greek: Δέλτα [ðelta], Dhelta) 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. In the Ancient Greek language, it represented a voiced dental plosive /d/, while in Modern Greek it represents a voiced dental fricative /ð/, hence it is pronounced as "th" in "that" or "this". it is romanised as either "D" or "Dh"

A river delta is so named because its shape approximates the upper-case letter delta (the shape is a triangle).

## The upper-case letter Δ

The upper-case letter Δ can be used to denote

• Change any type; (in science and engineering fields)
• Change; e.g. in
${y_2-y_1\over x_2-x_1} = {\Delta y \over \Delta x}$, the average change of y per unit x, i.e. the change of y over the change of x
$\Delta f = \sum_{i=1}^n {\frac{\partial^2 f}{\partial x_i^2}}$
$\Delta = b^2 - 4ac\,\!$

## The lower-case letter δ

The lower-case letter δ can be used to denote