Principal can also refer to money, normally a sum that was borrowed, but can be extended to refer to the amount you owe — hence principal and interest.

principal也可以用于指代金钱，通常指的是借贷的总额，或延伸至你所欠下的总额-也就是本金和利息。

youdao

In its Victorian incarnation, a tontine was an investment pool to which a group of speculators contributed capital, the principal and interest to be distributed in a lump sum years later to the last survivor.

FORBES: A 10% Annual Payout For Retirees