K denotes a family of programming languages designed by Arthur Whitney, which is sold by Kx Systems and Shakti and also supported by several independent implementations. K is an ASCII-only language influenced by Whitney's previous APL design A+. It has fewer primitives in part because it represents arrays as nested lists, unifying rank and depth, and encourages Scheme-like functional programming with first-class functions. Whitney presented the first K implementation (K0) in 1992, and soon founded Kx Systems to develop it further with versions numbering K1 through K6. K4 is now the implementation language for the time-series database kdb+ as well as derivative language Q. Whitney has developed further versions of K at Shakti, beginning with K7. Notable non-commercial implementations include Kona based on K3, and ngn/k and oK based on K6.
K's single-letter name parallels J, and Arthur Whitney had offered design input on J shortly before publishing K. Unlike J, K is often written k, without capitalization. Whitney has given various explanations of the choice of letter, including "keys to the kingdom".