The traditional abbreviation for a strikeout. A backwards K is often used to denote a called strikeout. Invented by Henry Chadwick by taking the "most prominent" letter of "struck" and reinforced by inference of "knockout" or "K.O." That connotation still exists, when the announcer says the pitcher "punched out" the batter, a play on words that also refers to "punching" a time clock.
A pitch thrown with no spin, traditionally thrown with the knuckles, but also with the fingertips. It tends to flutter and move suddenly on its way to the plate. Also refers to a batted ball that flutters "like a knuckleball."