Wiktionary
n. (context baseball English) A play where the catcher fails to stop a normally playable pitch and a runner advances.
WordNet
n. a pitch that the catcher should have caught but did not; allows a base runner to advance a base
Wikipedia
In baseball, a catcher is charged with a passed ball when he fails to hold or control a legally pitched ball that, with ordinary effort, should have been maintained under his control, and, as a result of this loss of control, the batter or a runner on base advances. A runner who advances due to a passed ball is not credited with a stolen base unless he breaks for the base before the pitcher begins his delivery.
A passed ball may be scored when a runner on first, second, or third base reaches the next base on a bobble or missed catch by the catcher, or when the batter-runner reaches first base on an uncaught strike three (see also Strikeout).
A closely related statistic is the wild pitch. As with many baseball statistics, whether a pitch that gets away from a catcher is a passed ball or wild pitch is at the discretion of the official scorer. Typically, pitches that are deemed to be ordinarily catchable by the catcher, but are not, are ruled passed balls, while pitches that get by the catcher that are thought to have required extraordinary effort by the catcher in order to stop them are wild pitches. If the pitch was so low as to touch the ground, or so high that the catcher has to rise out of his crouched position to get to it, or so wide that the catcher has to lunge for it, it is usually then considered a wild pitch and not a passed ball.
A scored run due to a passed ball is not recorded as an earned run. However, a scored run due to a wild pitch is recorded as an earned run.
Passed balls and wild pitches are considered to be part of the act of pitching rather than fielding. Thus they are kept as separate statistics and are not recorded as errors.
There tends to be a higher incidence of passed balls when a knuckleballer is pitching. The physics that make a knuckleball so difficult to hit make it similarly difficult to catch. While teams with a knuckleballer on their pitching staff often employ a special "knuckleball catcher" who is equipped with a knuckleball mitt, similar to a first baseman's glove, it is still extremely difficult to catch.
Usage examples of "passed ball".
The backstop, according to the rulebook, must be at least twenty feet from home plate, giving both the catcher and a runner at third a fair chance on a passed ball.
Jimmy came sweeping in behind, the others acting as a screen, and picked up the badly passed ball, kicking it toward Locklear.
To make it all the more exciting, Bogart hit safely, ran like a deer to third on Mullaney's grounder, which Wiler knocked down, and scored on a passed ball.
I watched the Reds blow a four-run lead, tie it up, and go into the thirteenth inning before they finally managed to scrape through with a very un-Reds-like win: a walk, a steal, a passed ball, and a sacrifice fly.
I have to admit that for me the toss to Elinor Vance was a passed ball.