All 3 Uses of

principal
in
The Merchant of Venice

And where thou now exacts the penalty,—Which is a pound of this poor merchant's flesh,—Thou wilt not only loose the forfeiture, But, touch'd with human gentleness and love, Forgive a moiety of the principal, Glancing an eye of pity on his losses, That have of late so huddled on his back, Enow to press a royal merchant down, And pluck commiseration of his state From brassy bosoms and rough hearts of flint, From stubborn Turks and Tartars, never train'd To offices of tender courtesy.†
Scene 4.1

principal = most important; or person that is most important

Definitions:

(1) (principal)
most important; or person who is most important
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
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

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