eta

Eta is the seventh letter of the Greek alphabet and a symbol used in various scientific, mathematical, and linguistic contexts. The name also appears in several abbreviations and designations.

In Greek script, uppercase Eta is Η and lowercase η. In Classical Greek, eta represented a long e sound; in Modern Greek, it corresponds to the close front unrounded vowel, roughly “ee” as in see. The letter is widely used as a symbol in mathematics, physics, and related fields.

In mathematics and physics, eta appears in several concepts: the Dedekind eta function in number theory; the eta meson in particle physics; eta-invariants in differential geometry; and eta-squared, a measure of effect size in statistics.

In astronomy, Eta is used in Bayer designations for stars, where a Greek letter indicates relative brightness within a constellation. An example is Eta Carinae in the Carina constellation.

As an acronym, ETA most commonly stands for estimated time of arrival. It also refers to Euskadi Ta Askatasuna, a Basque separatist group active in the late 20th century.