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  1. the fourth letter of the Greek alphabet (Δ, δ).
  2. the consonant sound represented by this letter.
  3. the fourth in a series of items.
  4. anything triangular, like the Greek capital delta (Δ).
  5. Mathematics. an incremental change in a variable, as Δ or δ.
  6. a nearly flat plain of alluvial deposit between diverging branches of the mouth of a river, often, though not necessarily, triangular: the Nile delta.
  7. (usually initial capital letter) a word used in communications to represent the letter D.
  8. (initial capital letter) Astronomy. a star that is usually the fourth brightest of a constellation: The fourth brightest star in the Southern Cross is Delta Crucis.
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Origin of delta

1350–1400; Middle English deltha < Latin delta < Greek délta; akin to Hebrew dāleth
Can be confuseddelta estuary


noun Military.
  1. the NATO name for a class of Soviet nuclear-powered ballistic missile submarine armed with 16 multi-warhead missiles.
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Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for delta


  1. the fourth letter in the Greek alphabet (Δ, δ), a consonant transliterated as d
  2. an object resembling a capital delta in shape
  3. (capital when part of name) the flat alluvial area at the mouth of some rivers where the mainstream splits up into several distributariesthe Mississippi Delta
  4. maths a finite increment in a variable
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Derived Formsdeltaic (dɛlˈteɪɪk) or deltic, adjective

Word Origin

C16: via Latin from Greek, of Semitic origin; compare Hebrew dāleth


  1. (foll by the genitive case of a specified constellation) usually the fourth brightest star in a constellation
  2. any of a group of US launch vehicles used to put unmanned satellites into orbit
  3. communications a code word for the letter d
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  1. a state of Nigeria, on the Niger river delta on the Gulf of Guinea. Capital: Asaba. Pop: 4 098 391 (2006). Area: 17 698 sq km (6833 sq miles)
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for delta


c.1200, Greek letter shaped like a triangle, equivalent to our "D," the name from Phoenician daleth "tent door." Herodotus used it of the mouth of the Nile, and it was so used in English from 1550s; applied to other river mouths from 1790.

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Online Etymology Dictionary, © 2010 Douglas Harper

delta in Medicine


  1. The fourth letter of the Greek alphabet.
  2. The fourth one in a series.
  3. A surface or part that resembles a triangle, such as the terminus of a pattern in a fingerprint or the shape of a muscle.
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  1. Of or characterizing the atom or radical group that is fourth in position from the functional group of atoms in an organic molecule.
  2. Of or relating to one of four closely related chemical substances.
  3. Relating to or characterizing a polypeptide chain that is one of five types of heavy chains present in immunoglobins.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

delta in Science


  1. A usually triangular mass of sediment, especially silt and sand, deposited at the mouth of a river. Deltas form when a river flows into a body of standing water, such as a sea or lake, and deposits large quantities of sediment. They are usually crossed by numerous streams and channels and have exposed as well as submerged areas.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.