A "K" is a strikeout.
Referring to a strikeout as a K goes all the way back to the mid-19th century -- when journalist Henry Chadwick began developing baseball's first scorecard. His original system used only single letters, and Chadwick couldn't use "S" for "struck" (the preferred term of the time period) because it had already been taken by "sacrifice." So instead, he decided to go with what he thought was the word's most memorable sound: the letter K.
Watch: The Nationals Park outfield videoboard tallies K's during Max Scherzer's 20-strikeout game.