List of baseball jargon

The following is an alphabetical list of selected unofficial terms, phrases, and other jargon used in baseball, and explanations of their meanings. See also baseball slang for slang in general usage that originated in baseball. For an exhaustive list, see The New Dickson Baseball Dictionary, by Paul Dickson.

J

;Jack

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:A Home Run, as in, "Hitting a jack" or "Jacking one out of here"

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;jam

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:As a verb, to throw a pitch far enough inside that the batter is unlikely to make good contact if he hits it. "The pitcher jammed the batter". As a noun, a situation where there are runners on base in scoring position, 1 or none out, and good hitters coming up. "The pitcher is in a jam."

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;Junior Circuit

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:The American League, so-called because it is the newer of the two major leagues.

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;junk

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:a sequence of several different pitches with not much velocity but a lot of movement. Throwing junk is usually a strategy of a pitcher who does not have a good fastball.

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