The exact meaning of principal can depend upon its context. For example:
"the principal idea" or "the principal performer" — the most important
"the principal of the school" — the person in charge
"a principal of a company" — an owner or person with controlling authority
"the principal in a transaction" (as contrasted to the agent) — the person on whose behalf an agent acts
But the pathos, the lesson, the moral of the great spectacle were lost upon the boy; he only thought of the conspicuousness of the principal character before the on-looking nations; his face lit with the thought, and he said to himself that he wished he could be that child, if it was a tame lion.
Chapter 5 (63% in)
There are no more uses of "principal" in The Adventures of Tom Sawyer.