A fastball is a type of pitch thrown by a pitcher.
The fastball.
Tim Hudson is a starting pitcher for the Atlanta Braves. His fastest pitch is the four seam fastball which he can throw at 93 mph.
In baseball, 'heat' is slang for fastball, so 'high heat' is a high fastball.
His fastball is his fastest and its in the range of 98-101.
2010 Chapman 105 mph fastball
The fastball is the pitch most thrown and most hit in baseball.
60mph
The top end on his fastball maxed out around 92 mph.
115mph Aroldis Chapman a left handed reliever for the Cincinnati Reds has the fastest pitch as verified by MLB at 105.1 MPH. It occurred September 24, 2010 in San Diego. It was a ball. He threw the pitch to Tony Gwynn Jr.
A slider is a pitch in baseball that breaks laterally and down. It is not thrown at the velocity that a fastball is thrown at, but its velocity is faster than a curve ball. The break on a slider is not as big as a curve ball. The slider is similar to a cut fastball, but will break more than a cut fastball.
In my view and other people may differ, a starting pitcher needs a good fastball, good curve and a slider. A pitcher sent in to pitch a closer, say one batter, a fast ball is best, realizing that facing a good fastball hitter better see some smoke on the fastball!! I realize I am sticking my head out on this one.