The eephus pitch.
It has also been called the blooper, the folly floater, the "LaLob", to name just three. It's a slow, high arcing curve ball that has been known to attain a height of 20 feet before descending. Rip Sewell was forced to come up with that one as the result of an injury, and he had good success with it---except for the time Ted Williams swung at that pitch and blasted it out of the ballpark and across the street into somebody's dining room. It's thrown straight overhand, which is the only way you can get the height required, and as has been noted, it's very easy to hit if you time it right. In recent years Orlando ("El Duque") Hernandez had a good degree of success with that pitch.
It's funny how the pitch got that name, when you consider that the originator of that term was none other than Lefty Gomez, and he used it differently---he meant put a little something extra on the ball.