noun: (grammar, usually in the plural) Any of the forms of a word which contain its stem(s) in the simplest form, or such a form that, when taken with all the other principal parts, allows the entire paradigm to be derived.
noun: (mathematics) A polynomial approximation of a power series, made up of monomials whose indices lie in the Newton diagram of the power series and which occur with the same coefficients as in the original power series.
noun: (mathematics) The portion of a Laurent series that has negative exponents.
noun: Used other than figuratively or idiomatically: see principal, part.
