principal part

noun[usually plural]6

1. •
   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 (showing various inflections), allows the entire paradigm to be derived.
2. •
   The portion of a Laurent series that has negative exponents.
3. •
   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.
4. •
   (no longer used) Any one of however many parts of speech regarded as the most fundamental or indispensable.
5. •
   (no longer used, rare) The noun, verb, or adverb.
6. •
   (no longer used) The noun or the verb.