Kappa (uppercase Κ, lowercase κ or ϰ; Greek: κάππα) is the 10th letter of the Greek alphabet, used to represent the /k/ sound in Ancient and Modern Greek. In the system of Greek numerals it has a value of 20. It was derived from the Phoenician letter kaph . 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 contemporary rendering into the Latin alphabet by ancient Romans. However, modern transliteration of the Greek language invariably use the letter "k". Compare Κυνοσκεφαλαι, Cynoscephalae (ancient placename); κυνος κεφαλαι, kunos kephalai (phrase), meaning "dog's heads". In mathematics, the kappa curve is named after this letter; the tangents of this curve were first calculated by Barrow (17th century).

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