A hold occurs when a relief pitcher enters the game in a save situation and maintains his team's lead for the next relief pitcher, while recording at least one out. One of two conditions must be met for a pitcher to record a hold: 1) He enters with a lead of three runs or less and maintains that lead while recording at least one out. 2) He enters the game with the tying run on-deck, at the plate or on the bases, and records an out.
As a statistic, holds are designed to credit late-relief pitchers who are not closers. For those pitchers, their primary job is to not relinquish the lead, while getting the ball to the next reliever in line. Every save opportunity in which a pitcher records at least one out will result in either a save, a blown save or a hold.
A pitcher cannot receive a win or a save in a game in which he records a hold. However, more than one relief pitcher can record a hold in a single game. It is also possible for a pitcher to receive a hold and a loss in the same game should he exit with the lead, only to see the runners he left on base score the tying and go-ahead runs.
The hold was invented in the 1980s by statisticians John Dewan and Mike O'Donnell as a means to quantify the effectiveness of relievers who aren't closers. The statistic was meant to value setup men -- who traditionally pitch the eighth inning before the closer pitches the ninth.