Webster's defines principal, as a noun, in the relevant senses as:
1 : a person who has controlling authority or is in a leading
position: such as . . .
a : a chief or head man or woman . . .
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 . . .
e : the person primarily or ultimately liable on a legal obligation .
In the context the question indicates, the reference is to the chief decision maker who is usually an owner as well as someone with decision making power, often without regard to whether someone is formally an officer of the company.
In the case of a single own corporation whose single owner is also the most senior executive or corporate officer, the person would be the single owner, and the title would ordinarily be "CEO" or "President" or "Founder" or "CEO and Chairman of the Board". Normally, the title would be reflected in some corporate document, although not necessarily one that is publicly filed.
Your title is up to you and can be anything you choose within reason.