Ace--a team's best starting pitcher.
Alley--the section of the outfield between the outfielders.  also:  "gap".
Around the horn--a double play going from 3rd to 2nd to 1st.
Backdoor slider--a pitch appears to be out of the strike zone, but then breaks back over the plate.
Bag--a base.
Baltimore chop--a ground ball that hits in front of home plate (or off of it) and takes a large hop over the infielders head.
Bandbox--a small ballpark that favors hitters.
Bang-bang play--a play in which the baserunner hits the bag a split-second before the ball arrives or vice-versa.
Basket catch--when a fielder catches a ball with his glove near belt level.
Bronx cheer--when the crow boos.
Brushback--a pitch that nearly hits a batter.
Bush--also "bush league".  an amateur play or behavior.
Can of corn--an easy catch by a fielder.
Caught looking--when a batter is called out on strikes.
Cellar--last place.  also:  "basement".
Cheese--also "good cheese."  refers to a good fastball.
Chin music--a pitch that is high and inside.
Circus catch--an outstanding catch by a fielder.
Closer--a team's relief pitcher who finishes the game.
Cutter--a cut fastball (one with a late break to it).
Cycle--when a batter hits a single, double, triple and home run in the same game.
Circle--when a hitter gets no hits or walks for the game.   his game line-score will be all zeros.  ie:  circles.  "he hit for a circle today".
Dinger--a home run.
Dish--home plate.
Fireman--a team's closer or late-inning relief pitcher.
Fungo--a ball hit to a fielder during practice.  it's usually hit by a coach using a "fungo bat".  which is longer and thinner than a normal bat.
Gap--also "alley."  a ball hit here is a "gapper."
Gopher ball--a pitch hit for a home run, as in "go for."
Heat--a good fastball.  also:  "heater."
High and tight--referring to a pitch that's ups in the strike zone and inside on a hitter.  also known as "up and in".
Hill--pitcher's mound.
Homer-- a home run.  other terms include:  blast, dinger, dong, four-bagger, four-base knock, moon shot, tape-measure blast and tater.
Hot corner-- 3rd base.
In the hole--the batter after the on-deck hitter.
Jam--when a hitter gets a pitch near his hands, he is "jammed".  also when a pitcher gets himself in trouble, he is in a "jam".
Leather--refers to how good a player plays defensively or handles the glove.  ex:  "he flashed some leather on that play".
Meatball--an easy pitch to hit, usually right down the middle of the plate.
Mendoza line--a batting average of around .200.
Moon shot--a very long, high home run.
Nail down--as in "nail down a victory".  refers to a relief pitcher finishing off the game.
On the screws--when a batter hits the ball hard.  also:  "on the button".
Painting the black--when a pitcher throws the ball over the edge of the plate.
Pea--a ball traveling at high speed, either batted or thrown.
Pepper--a common pre-game exercise where one player bunts brisk grounders and line drives to a group of fielders who are standing about 20 feet away.  the fielders try to throw it back as quickly as possible.  the batter hits the return throw.  (some ballparks ban pepper games because wild pitches could land in the stands and injure spectators).
Pick--a good defensive play by an infielder on a ground ball.  also, a shortened version of "pick-off".
Pickle--a rundown.
Punchout--a strikeout.
Rhubarb--a fight or scuffle.
Ribbie--another way of saying RBI.  also:  "ribeye".
Rope--a hard line drive hit by a batter.  also:  'frozen rope".
Rubber game--the deciding game of a series.
Run-down--when a  baserunner gets caught between bases by the fielders.
Ruthian--with great power.
Seeing-eye single--a soft ground ball that finds its way between infielders for a base hit.
Setup man--a relief pitcher who usually enters the game in the 7th or 8th inning.   he "sets-up"  the closer.
Shoestring catch--a running catch made just above the fielder's shoetops.
Southpaw--a left-handed pitcher.
Sweet spot--the part of the bat just a few inches from the barrel.
Table setter--batter whose job is to get on base for other hitters to drive him in.  usually a leadoff or #2 hitter.
Tape-measure blast--an extremely long home run.
Tater--a home run.
Texas League-- a bloop hit that drops between an infielder and outfielder.
Tools of ignorance--catcher's equipment.
Touch 'em all--hitting a home run  (touching all the bases).
Twin killing--a double play.
Uncle Charlie--curve ball.
Utility player--a player who fills in at many positions.
Wheelhouse--a hitter's power zone.  usually a pitch waist-high over the heart of the plate.
Wheels--a ballplayers legs.
Whiff--a strikeout.
Yakker--curve ball.