"(It's) a curve ball that doesn't give a damn. - Jimmy Cannon
A knuckleball (aka knuckler, flutter pitch, butterfly pitch) is a pitch that is thrown very slowly (55-70 mph) with very little spin. A pitcher that throws a knuckleball is called a knuckleballer. Unlike other pitches, whose trajectories can be predicted from their speed and spin, the knuckleball is completely unpredictable. It can actually break in one direction and then turn around and break the other way. This makes it very difficult to hit (DIPS suggests that knuckleballers are the only group of pitchers with a consistent ability to decrease opponents' batting average on balls in play) and to catch (catchers of knuckleball pitchers are routinely charged with enormous numbers of passed balls).
"The way to catch a knuckleball is to wait until the ball stops rolling and then to pick it up. - Bob Uecker
Contrary to its name, most pitchers actually grasp the knuckleball with their fingernails or fingertips, not with their knuckles, though young pitchers may find it easier to throw it with their knuckles. The knuckleballer pushes the ball with his fingers as he releases it, which prevents it from getting much spin or much speed. An ideal knuckleball rotates less than one half rotation between the pitcher's hand and the plate. Because the ball rotates so slowly, the uneven placement of its stitches in the airflow causes an uneven drag on the ball and hence a sideways motion. When the stitches rotate into or out of the airflow, they can cause a sudden reversal of the sideways force and consequently of the direction of break.
Because it is so difficult to master, many knuckleball pitchers specialize in the knuckleball to the near exclusion of other pitches. John Smoltz is about the only recent pitcher who ever threw a knuckleball as part of an ordinary repertoire, and he has done so very rarely. This exclusivity was not always the case; before World War II many pitchers would use the knuckleball occasionally, often as a form of change-up.
The pitcher credited with inventing the knuckleball is usually Toad Ramsey, who did it out of necessity because an injury suffered while working as a bricklayer prevented him from bending one of his fingers on his throwing hand (he was lefthanded, something that is very rare among knuckleball pitchers). While the pitch he threw was more of a knuckle-curve than a true knuckleball, he established the fact that such a slow pitch with a strong and unpredictable break could be extremely effective against event the best hitters.
- Michael Clair: "The story behind the first knuckleball may surprise you: An origin story fitting for such a strange pitch", mlb.com, March 28, 2020. 
- david Gendelman: "Unpredictable and unmanly: baseball's fear of the knuckleball", The Guardian, September 23, 2016. 
- Paul Newberry (Associated Press): "Baseball without the knuckler would be a real shame", USA Today Sports, October 2, 2017. 
- Dan Schlossberg: "Dan’s Dugout: Return of the Knuckleball", Latino Sports, May 12, 2016.