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In baseball or softball, a strikeout or strike out (denoted by SO or K) occurs when a batter receives three strikes during his time at bat. A strikeout is a statistic recorded for both pitchers and batters. Pitchers want to throw as many strikeouts as possible, while batters attempt to minimize striking out themselves. While strikeouts are associated with dominance on the part of the pitcher, it is recognized that the style of swing that generates home runs also leaves the batter somewhat susceptible to striking out. Some of the greatest home run hitters of all time — such as Reggie Jackson and Sammy Sosa — were notorious for striking out.
A pitcher receives credit for (and a batter is charged with) a strikeout on any third strike, but a batter is out only if any of the following is true:
If the third strike is not caught and there are two outs, or fewer than two outs and no baserunner on first, the batter becomes a runner (except in a foul bunt situation). Thus, it is possible for a batter to strike out, but still reach base safely if the catcher fails or is unable to catch the third strike cleanly and cannot tag out the batter or force him out at first base (in Japan this is called furinige (振り逃げ), i.e. swing and escape). In MLB, Major League Baseball, it is known as a uncaught third strike. As a result, pitchers have occasionally been able to record four strikeouts in one half-inning.
In baseball scorekeeping, a swinging strikeout is recorded as a K, or a K-S. A strikeout looking (where the batter does not swing at a pitch that the umpire then calls strike three) is sometimes scored with a backwards K.
The use of "K" for a strikeout was invented by Henry Chadwick, a newspaper journalist who is widely credited as the originator of the box score and the baseball scorecard. Both the box score and scorecard persist largely unchanged to this day, as the game itself is largely unchanged except for the number of balls and strikes allowed to the pitcher and batter. The letter "S" was used for "sacrifice" so Chadwick decided to use "K", being the last letter in "struck". Chadwick also invented many other baseball scoring abbreviations, such as using numbers to designate player positions, progressing from the pitcher , catcher , through the infield, with the shortstop  counted after the basemen, to the right fielder .
That Chadwick first established the convention of using the "K" abbreviation is well-founded, with reliable and authentic primary materials surviving (see citation above). Those unaware of Chadwick's contributions have speculated that "K" was derived from the 19th century pitcher Matt Kilroy's last name. If not for the evidence supporting Chadwick's earlier use of "K", this speculation would be reasonable: Kilroy did much to raise the prominence of the strikeout, setting an all-time record of 513 strikeouts in 1886, only two years after overhand pitching was permitted. Kilroy's record, however, is forever confined to its era: the pitcher's mound during his record-setting season was only from the batter; it was moved to its current distance of 60'6" in 1893. The modern record (1901-) is 383 strikeouts, held by Nolan Ryan, one better than Sandy Koufax's 382.
Although some people use "K" to record pitchers' strikeouts, "SO" is the official abbreviation used by Major League Baseball.
In addition, "K" is still commonly used by fans and enthusiasts for purposes other than official record-keeping. In one baseball ritual, fans at the ballpark who are seated in view of the batter (and the television cameras) attach a succession of small "K" signs to the nearest railing, one added for every strikeout notched by the home team's pitcher. As is traditional for those who keep a record of the game on paper, the "K" is placed backwards in cases where the batter strikes out looking. Virtually every televised display of a major league game in which a pitcher registers a high number of strikeouts (7 or 8) will include a shot of a fan's strikeout display, and if the pitcher continues to strike out batters, the display often will be shown following every strikeout. In the event that a known "strikeout pitcher" is on the mound, the strikeout display will be televised from the beginning.
Three balls being struck at and missed and the last one caught, is a hand-out; if not caught is considered fair, and the striker bound to run. This is essentially the same rule in use today, with the addition of the called strike (1858) and the provision that the batter is automatically out if there are fewer than two out and a runner on first. In 1880, the rules were changed to specify that a third strike had to be caught on the fly. In 1887, the number of strikes for an out was changed to four, but promptly changed back to three the next season.
A swinging strikeout is often called a whiff and a batter who is struck out by a fastball is often said to have been blown away. A batter who strikes out on a swung third strike is said to have fanned (as in a fanning motion), whereas if he takes a called third strike it is called a punchout (describing the plate umpire's punching motion on a called third strike), but sometimes these descriptive words are used generally as synonyms for strikeouts, irrespective if they were calling or looking (e.g. Beckett's punched out nine batters tonight or Ryan Howard's been fanned six times in this series). On a called third strike, it is said that the batter was caught looking or that he looked at a strike. Typically, a called third strike can be somewhat more embarrassing for a batter, as it shows that he was either fooled by the pitcher, or even worse, had a moment of hesitation. For example, Carlos Beltran was caught looking at strike 3 to end the 2006 NLCS, and the season, for the New York Mets. Sports commentators have also been known to refer to it as browsing if the batter did not move his bat at all.
A pitcher is said to strike out the side when he retires all three batters in a half inning by striking them out. A batter that takes the third strike looking, especially on a breaking pitch, such as a slider or a curveball, that appears to be out of the strike zone but drops in before the batter can get the bat off his shoulders, can be said to have been frozen.
In slang, when a batter strikes out three times in a game, he is said to have completed a hat trick. If he strikes out four times, it is known as a golden sombrero. He receives a platinum sombrero if he strikes out five times, also known as the Olympic Rings. Striking out six times is a rare occurrence, which in the history of major league play has only been accomplished in extra innings games - Sam Horn is one of the distinguished few to achieve this feat.
Some pitchers who specialize in strikeouts have acquired nicknames including the letter "K". Cincinnati Reds closer Francisco Cordero is known as "Koko", Dwight Gooden was known as "Doctor K" (back-referencing basketball star Julius Erving a.k.a. "Dr. J"). Francisco Rodriguez is known as "K-Rod". Roger Clemens has taken the "K" name to an extreme, naming his four sons Koby, Kory, Kacy, and Kody. Hall of Fame strikeout artist Sandy Koufax of the Los Angeles Dodgers coincidentally has a last name starting with "K", and in his call of Koufax's perfect game in 1965, Dodgers announcer Vin Scully included a comment that Koufax's name "will always remind you of strikeouts".