Kappa carries a scattered set of modern technical meanings, none connected to the letter's ancient Semitic root kaph, meaning the palm of the hand, discussed elsewhere in this atlas. Physicists use kappa for a variety of constants depending on context, including the spring constant in Hooke's law and measures of curvature in geometry and general relativity; statisticians use Cohen's kappa, introduced by the psychologist Jacob Cohen in 1960, as a standard measure of agreement between two independent raters that corrects for the agreement expected by pure chance, a tool still widely used in fields from medicine to content moderation research.
Kappa is also, unusually among Greek letters, one of the more graphically stable across the alphabet's history, its angular shape essentially unchanged from the Phoenician kaph it descends from, since, unlike letters such as beta or sigma, its ancient consonant value never shifted or split into rival forms. Astronomers, following the Bayer star-naming convention of 1603, use Kappa to mark a constellation's eleventh- or twelfth-brightest star, continuing the sequence used for the alphabet's other letters. In the Milesian numeral system, discussed elsewhere in this atlas, kappa holds the value twenty, a role with no bearing on any of its later scientific or statistical uses.