A two-seam fastball. A pitch thrown with high velocity like a regular fastball, but typically with more movement, often downwards like a sinker. Named after the positions of the pitcher's index and middle fingers, which are held along the baseball's two seams where they are closest together.
"Jeff Samardzija and Doug Fister throw some of the best two-seamers in the game. Samardzija throws them faster, but Fister gets more ground-ball outs."
"Why are you telling me this?"
"Well, I thought maybe you'd be interested, since you're looking it up Urban Dictionary."