delta

noun

del·​ta ˈdel-tə 

plural deltas

1

: the 4th letter of the Greek alphabet see Alphabet Table

2

: something shaped like a capital Greek delta

especially, geology : the alluvial deposit at the mouth of a river

the Mississippi Delta

3

mathematics : an increment of a variable

—symbol Δ

4

: delta wave

During deep sleep, the brain operates in delta wave frequency. But it's difficult to go from beta to theta then delta without first switching down to a relaxed alpha state.

—Daily Mail (London)

5

or less commonly Delta : delta variant

As SARS-CoV-2 continues to spread among humans, natural selection and adaptation will result in more variants that could plausibly be more transmissible than delta.

—Suresh V. Kuchipudi

deltaic

del-ˈtā-ik 

adjective

Examples of delta in a Sentence

Recent Examples on the Web

Last September, a paper in the journal Nature reported that the Perseverance Mars rover had discovered a curious rock in a dry delta that was once part of a river. —Jeffrey Kluger, Time, 1 June 2026

Lower flows mean less sediment flushing downstream, accelerating the sinking of the Louisiana delta. —Trevor Hughes, USA Today, 31 May 2026

The massive delta is revered not only for its capacious water supply and agricultural support, but also its famed Delta breeze that often brings relief from stretches of hot weather. —Camila Pedrosa, Sacbee.com, 29 May 2026

But with that comes the risk of accidents, hazards and drownings in the delta and local rivers. —Carmela Karcher, CBS News, 27 May 2026

Word History

Etymology

Middle English deltha, from Greek delta, of Semitic origin; akin to Hebrew dāleth daleth

First Known Use

12th century, in the meaning defined at sense 1

Phrases Containing delta

• delta agent
• delta hepatitis
• delta ray
• delta variant
• delta virus
• delta wave
• delta wing
• hepatitis delta virus
• Kronecker delta

Kids Definition

delta

noun

del·​ta

ˈdel-tə

1

: the 4th letter of the Greek alphabet

—Δ or ƍ

2

: something shaped like a capital Δ

especially : the triangular or fan-shaped piece of land made by deposits of mud and sand at the mouth of a river

deltaic

del-ˈtā-ik

adjective

Medical Definition

delta

1 of 2 noun

del·​ta ˈdel-tə 

1

: the fourth letter of the Greek alphabet

—symbol Δ or ƍ

2

: any of various things felt to resemble a capital Δ

especially : the triangular terminus of a pattern in a fingerprint formed either by bifurcation of a ridge or by divergence of two ridges that are parallel beyond it

3

: delta wave

4

also Delta : delta variant

delta

2 of 2 adjective

variants or ƍ-

1

: of or relating to one of four or more closely related chemical substances

the delta chain of fetal hemoglobin

ƍ-yohimbine

—used somewhat arbitrarily to specify ordinal relationship or a particular physical form and especially one that is allotropic, isomeric, or stereoisomeric (as in ƍ-benzene hexachloride)

2

: fourth in position in the structure of an organic molecule from a particular group or atom

also : having a structure characterized by such a position

ƍ-hydroxy acids

ƍ-lactones