Principal
Principal is the sum borrowed or invested, separate from any interest charged or earned on it. On a loan it is the balance still owed, and only the part of a payment left after the interest due reduces it.
Interest is always a percentage of principal, so the principal is the figure every other number on a loan is derived from. Multiply the outstanding principal by the periodic rate and you have the interest due for that period. Pay more than that amount and the remainder comes off the principal, which makes the next period's interest smaller. Pay less and the shortfall can be added to the balance, which makes it larger.
That mechanism is why an amortisation schedule looks so lopsided at the start. Early in the term the balance is near its largest, so most of a fixed payment is consumed by interest and only a thin slice reaches the principal. The loan payment calculator shows the split period by period for a given rate and term.
The common error is reading a payment as principal reduction. A payment reduces the balance by the payment minus the interest due, never by the whole payment. It is also why lenders treat an extra sum sent as a principal-only payment differently from one that simply pays the next instalment early: the first shortens the loan, the second only moves a due date.