In Whyte notation, a Pacific is a 4-6-2 wheel arrangement.
The three numbers count wheels from front to rear: four leading wheels on two axles, six coupled driving wheels on three axles, and two trailing wheels on one axle. The leading truck helped a locomotive remain stable at speed, while the trailing truck supported a larger firebox behind the driving wheels.
The type became widely known as Pacific after a Baldwin-built locomotive was shipped across the Pacific Ocean to New Zealand. New Zealand’s Q class, introduced in 1901, helped establish the arrangement, although the first Pacific in the United States is associated with the Lehigh Valley Railroad. The name was later used internationally for powerful express-passenger locomotives.
A common mix-up is 4-6-4, called a Hudson or Baltic, which has four trailing wheels rather than two. Another is 4-4-2, the Atlantic type, with only four driving wheels. LNER Mallard was a famous British Pacific.