The yips is a slangish term used to describe a condition in which a pitcher loses the ability to throw a ball in the strike zone, because his mechanics have gone completely out of whack. While a physical problem is sometimes the cause, it is before all a mental issue, as a movement that normally comes completely naturally is suddenly impossible to reproduce without overthinking things. A pitcher battling the condition normally retains his velocity, but cannot throw the ball consistently over the plate, with a huge and seemingly inexplicable rise in walks, hit batsmen and wild pitches.
It is extremely difficult to overcome the condition, and most pitchers eventually give up in frustration and move on to other things. Among famous cases are Steve Blass, whose career was ended by the condition (and it is often referred to as Steve Blass disease), Mark Wohlers, Rick Ankiel (who reinvented himself as a position player and was able to resume his career that way), and Daniel Bard. Before the condition was documented with Blass, it already existed, with author Pat Jordan and Joe Sparma having fallen victim to it.
The condition sometimes affects position players as well, and while it is not as immediately damaging for them, it is still career-threatening. Second baseman Steve Sax was a victim in the early 1980s, and catcher Mackey Sasser a decade later. The condition forms an important plot point in the baseball novel The Art of Fielding when it affects a top college shortstop prospect.
Because there is nothing physically wrong with the player, the yips are considered highly mysterious, and make the sufferer a sort of pariah. Pat Jordan describes in his autobiography, A False Spring how other players would shun him as if he had communicable disease.