delta
Pluraldeltas
Pluraldeltas
Formsdeltas, deltaing, deltaed
- 1A landform at the mouth of a river, where the river empties into a body of water such as a sea or lake.
The river splits into dozens of channels as it crosses the delta to the gulf.
Rising seas threaten the farms and villages of the Nile Delta.
- 2The fourth letter of the modern Greek alphabet, written Δ as a capital and δ in lowercase.
The fraternity's sign shows a large delta between two other Greek letters.
In the Greek alphabet, delta comes after gamma and before epsilon.
Note
Greek borrowed the letter's name from a Phoenician word for "door", which goes back to Proto-Semitic *dalt-. The letter name dalet comes from the same source, so delta and dalet are one word that reached English by two different roads.
- 3The symbol Δ, or the change in a quantity that the symbol stands for.
The engineers measured the pressure delta across the valve every hour.
A large delta between indoor and outdoor temperatures drives heat through the walls.
Similardifference, differential
Note
In a formula the capital letter goes in front of whatever changed: Δx, read "delta x", is the final value of x minus the starting one. Rocketry and orbital mechanics speak of ΔV, said "delta vee", for a change in velocity. The letter was probably chosen as the Greek d, for difference.
- 4Ellipsis of delta variant, a variant of the SARS-CoV-2 virus.
Hospitals filled up again as delta spread through communities with low vaccination rates.
Researchers compared how easily delta and earlier variants passed between members of a household.
Note
Dropping the word variant turns the phrase into something like a name: delta spread, people caught delta, with no article, while the full phrase keeps one, as in the delta variant. The full phrase also appears with a capital D, as Delta variant, and the strain it names is the one scientists label B.1.617.2.
- 5Alternative letter-case form of Delta from the NATO/ICAO Phonetic Alphabet.
The caller read out the license plate as delta, echo, four, seven.
Ground control cleared the jet to taxi via delta to the main runway.
Note
A word built to survive a bad radio line: in the international spelling alphabet each letter has a code word that is hard to mishear, and D has delta, so a name like Dan goes out as delta, alpha, november. The official tables print the code words capitalized, as Delta; in running text the word is often written in lowercase.
- 6The full set of differences between two versions of the same file.
Instead of copying the whole database each night, the backup tool keeps a series of deltas.
Reading the delta line by line, he spotted a typo in the new configuration.
Similardiff
Note
Version-control systems lean on deltas to keep a file's history compact: one version is stored whole, and the others are rebuilt by applying deltas to it in order. The same idea keeps many software updates small, since only the delta has to travel. The word is countable, so a running history is a series of deltas.
- 7The rate at which an option's value changes in relation to changes in the price of the underlying asset.
With a delta of 0.5, the call option should gain about fifty cents for every dollar the stock rises.
Market makers adjust their stock holdings throughout the day to keep their portfolio's delta in check.
Note
Options traders use it to see which way a position leans with the market: a call option has a positive delta, since it gains when the asset rises, and a put has a negative one. Balancing holdings so that their deltas cancel out, a practice called delta hedging, shields a portfolio from small moves in the asset's price.
- 8An effect that is small but large enough to notice.
Moving the check out of the loop improved performance by delta, enough to show up in the benchmarks.
The patch raises memory use by delta, so we flagged it in the release notes.
Note
Programmers took this use from mathematics, where a lowercase delta often stands for a small quantity. It often goes without an article, as in slower only by delta. Its partner epsilon sits a step lower: an effect of epsilon is too small to matter, while an effect of delta can still be measured.
- 9To compare two versions of a file to find where they differ, which shows where a programmer has made edits.
The build script deltas each source file against the last release before packaging the update.
Developers delta the configuration files after every merge to catch accidental edits.
Note
The verb belongs to source-file tools such as SCCS: a tool deltas a file, usually against an earlier version of itself, and what comes out is the set of differences, ready to be stored or reviewed. Its object is the file, not the edits, so a programmer deltas the configuration rather than the changes made to it.
- 10The closed figure made by joining three coils or circuits end to end in sequence, especially in a three-phase system.
The transformer's primary windings are connected in delta, while the secondary windings use a wye arrangement.
The technician traced the fault to a burned-out section of the generator's delta winding.
Note
The name is a picture: on a circuit diagram the three coils joined end to end form a triangle like the capital Δ. The word often works in front of another noun, as in delta winding, delta connection or delta current, and after in, as when windings are connected in delta. Its usual counterpart is the wye, or star, connection, whose three coils meet at a single point.
- 11A star typically surpassed in brightness by only three others in its constellation.
Astronomers tracked the brightness of the constellation's delta over several nights.
Unlike most deltas, this star outshines the gamma of its constellation.
Note
Stars in a constellation are lettered with the Greek alphabet roughly in order of brightness, which is why delta usually, but not always, marks the fourth. In a star's full name the letter comes first and the constellation's Latin name follows in the genitive.
- 12In the US Space Force, a unit nominally headed by a colonel, equivalent to an Air Force operations wing or an army regiment.
After the reorganization, the base's squadrons were grouped under a single delta.
Officers from three deltas met to coordinate support for the satellite launch.
Note
Squadrons are grouped under a delta, which sits in the middle of the US Space Force's chain of command and reports in turn to a larger command above. The name is the Space Force's own: a reader moving between services will meet the same rung as an operations wing in the Air Force and a regiment in the army.
- 13A cargo bike with a single wheel in front and two wheels at the back.
The courier traded her two-wheeler for a delta, which keeps heavy crates steady over the rear axle.
Deltas are easier to balance at low speed than two-wheeled cargo bikes, but they take up more room on a bike path.
- 14A value given in delta notation that expresses the relative abundances of isotopes.
The oxygen-isotope deltas of the fossil shells shift sharply at the boundary between the two layers.
Each sample's carbon delta was reported relative to the international reference standard.
Note
Isotope work measures against a yardstick instead of counting: a sample's isotope ratio is compared with that of a reference standard, and the difference is given in parts per thousand (‰). The value is written with the lowercase δ and the heavier isotope, as in δ¹³C or δ¹⁸O, and a negative delta means the sample is poorer in that heavier isotope than the standard.
- 15Any of the four baryons made of up and down quarks with a combined spin of 3/2, written Δ⁺⁺, Δ⁺, Δ⁰ and Δ⁻.
When a pion strikes a proton at the right energy, the pair can briefly form a delta that decays almost at once.
The four deltas carry electric charges ranging from plus two down to minus one.
Note
These particles are close relatives of the proton and neutron: built from the same up and down quarks, but with all three spins lined up, which raises the total to 3/2. Their charge follows the quark mix, from uuu for Δ⁺⁺ down to ddd for Δ⁻, and none of them lasts: each decays into a nucleon and a pion.
- 16The angle formed at the center of a circle by lines drawn to the two ends of a circular arc.
With the radius and the delta known, the crew could calculate the length of the curve.
The plans list a delta of 32 degrees for the curve on the bridge approach.
Note
Surveyors laying out a curve on a road or railroad mark this angle with the capital Δ. It equals the angle through which the route turns between the two straight stretches the curve joins. Together with the radius it fixes the rest of the curve's geometry: its length, its tangent distances and its chord.
- 17In cryptanalysis, to compute how each character of an enciphered text differs from the character a fixed number of places earlier.
Analysts would delta the intercepted stream so that regularities in the machine's wheels showed through.
Working by hand, the team deltaed each new intercept and tallied the results on squared paper.
Note
The operation turns a stream of characters into a stream of changes, and in that form regularities hidden in a cipher machine's key can show through. Its home is the breaking of the Lorenz cipher traffic known as Tunny, where Turing saw that deltaing exposed what was otherwise concealed. The participle is written both deltaing and delta-ing, and the past form is deltaed, keeping the final a.
Showing meanings only.
Practice
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Origin
From Ancient Greek δέλτα (délta), borrowed from a Phoenician word for "door" and ultimately from Proto-Semitic *dalt-; it is a doublet of dalet. The river sense comes from the triangular shape of the capital Greek letter Δ, and the USSF sense from the delta wing, the force's symbol, a triangular wing shaped like Δ.