Definition of "koppa"

The 18th Ancient Greek letter in early versions of the Greek alphabet, Ϙ (lowercase ϙ).  examples

The Greek numeral Ϟ (lowercase ϟ), descended from the ancient letter and having the numerical value of 90.  examples

The archaic Cyrillic numeral Ҁ (lowercase ҁ), descended from the ancient Greek letter and having the numerical value of 90.  examples