A type of pitch.
According to Walter Alston's instructional book:
Sinking Fast Ball (with the seams). The "sinking" fastball is released with an over-the-top, then outside-in, flip of the wrist. For a right-handed pitcher, this ball drives down and in on a right-handed hitter, and down and away from a left-handed swinger. In releasing the ball, the pitcher has to turn the ball over at the last moment, placing more pressure on the index finger.
This fast ball is gripped with the middle finger and forefinger curled snugly, not tightly, along the two parallel short seams. The thumb underneath pinches across the short seams on the lower half of the baseball.
Pitchers that rely on sinkers tend to pitch to contact, meaning their strikeout rates are rather low.