When subtracting two numbers, regrouping means starting at the ones column and borrowing a value from the next column as one moves to the left until the subtraction is completed. Regrouping and borrowing are words that mean the same thing when subtracting.
The Bessemer process was the first inexpensive industrial process for the mass-production of steel from molten pig iron prior to the open hearth furnace. The key principle is removal of impurities from the iron by oxidation with air being blown through the molten iron.
It was invented in 1851 by William Kelly but was was independently invented by Henry Bessemer in 1855 (and Bessemer took out a patent on the process).