From BR Bullpen

A sinker or a sinkerball is 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.

Jon Garland, Chien-Ming Wang, Brandon Webb, and Fausto Carmona are a few notable sinker ball pitchers. In the 2010s, it has largely become a specialty pitch used principally by lefthanded short relievers, its advantage being that it is equally effective against righthanded and lefthanded batters, and against which it is difficult to make hard contact.

Source: The Neyer/James Guide to Pitchers. - Bill James and Rob Neyer