Definitions:
first in order of importance; main

denoting an original sum invested or lent

the most important or senior person in an organization or group

a sum of money lent or invested, on which interest is paid

a person for whom another acts as an agent or representative

the person directly responsible for a crime.

a main rafter supporting purlins.

an organ stop sounding a main register of open flue pipes typically an octave above the diapason