Sometimes referred to as principal loan amount. The loan amount you borrowed plus any interest.

Merriam-Webster Online Dictionary
principal (adjective)
1.
most important, , or influential - consequential chief the principal ingredient the region's principal city
2.
of, relating to, or constituting principal or a principal - (see principal) principle
principal (noun)
1.
a person who has controlling or is in a leading position as - authority
a) a or head man or woman - chief
b) the chief executive officer of an educational institution
c) one who engages another to act as an agent subject to general control and instruction , specifically the person from whom an agent's authority derives
d) the chief or an actual participant in a crime
e) the person primarily or ultimately on a legal obligation - liable
f) a leading performer - star
2.
a matter or thing of importance as - primary
a) (1) a capital sum earning interest, due as a debt, or used as a fund
(2) the of an estate, portion, , or bequest - corpus devise
b) the construction that gives shape and strength to a roof and is usually one of several - trusses , broadly the most important member of a piece of framing principle