thumb|right|75px|Variant kappa thumb|right|Greek word καί written with a handwritten variant of kappa, from the Byzantine period Kappa  (uppercase Κ, lowercase κ or cursive ϰ; , káppa) is the 10th letter of the Greek alphabet, used to represent the [] sound in Ancient and Modern Greek.
In the system of Greek numerals,  has a value of 20.
It was derived from the Phoenician letter kaph 20px.
Letters that arose from kappa include the Roman K and Cyrillic К.
Greek proper names and placenames containing kappa are often written in English with "c" due to the Romans' transliterations into the Latin alphabet: Constantinople, Corinth, Crete.
All formal modern romanizations of Greek now use the letter "k", however.
The cursive form  is generally a simple font variant of lower-case kappa, but it is encoded separately in Unicode for occasions where it is used as a separate symbol in math and science.
In mathematics, the kappa curve is named after this letter; the tangents of this curve were first calculated by Isaac Barrow in the 17th century.
Symbol
Lowercase (κ)
;Mathematics and statistics
In graph theory, the connectivity of a graph is given by κ.
In differential geometry, the curvature of a curve is given by κ.
Kappa statistics such as Cohen's kappa and Fleiss' kappa are methods for calculating inter-rater reliability.
;Physics
In cosmology, the Einstein gravitational constant is denoted by κ.
In physics, the torsional constant of an oscillator is given by κ.
In physics, the coupling coefficient in magnetostatics is represented by κ.
In fluid dynamics, the von Kármán constant is represented by κ.
In thermodynamics, the compressibility of a compound is given by κ.
;Engineering
In structural engineering, κ is the ratio of the smaller factored moment to the larger factored moment and is used to calculate the critical elastic moment of an unbraced steel member.
In electrical engineering, κ is the multiplication factor, a function of the R/X ratio of the equivalent power system network, which is used in calculating the peak short-circuit current of a system fault.
κ is also used to denote conductivity, the reciprocal of resistivity, rho.
;Biology and biomedical science
In biology, kappa and kappa prime are important nucleotide motifs for a tertiary interaction of group II introns.
In biology, kappa designates a subtype of an antibody component.
In pharmacology, kappa represents a type of opioid receptor.
;Psychology and psychiatry
In psychology and psychiatry, kappa represents a measure of diagnostic reliability.
;Economics
In macroeconomics, kappa represents the capital-utilization rate.
Uppercase (Κ)
;History
In textual criticism, the Byzantine text-type (from Κοινη, Koine, the common text).
;Mathematics and statistics
In set theory, kappa is often used to denote an ordinal that is also a cardinal.
;Chemistry
In chemistry, kappa is used to denote the denticity of the compound.
In pulping, the kappa number represents the amount of an oxidizing agent required for bleaching a pulp.
Character encodings
Mathematical Kappa
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.
References
