It was from frequency analysis lecture and the professor mentioned but I cannot recall. The K value for English was .067, German was .076. The lowest K was Russian which was .056. When the frequency is uniform among 26 letters, then K is .038. What is the complete word that K stands for?
It probably refers to the Index of coincidence, or more accurately the un-normalized index of coincidence, referred to in the Wikipedia article as "kappa-plaintext".