principal part

Short definition from Wiktionary

Noun

1. 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. 2
   Any one of however many parts of speech regarded as the most fundamental or indispensable.
3. 3
   Any one of however many parts of speech regarded as the most fundamental or indispensable: The noun, verb, or adverb.
4. 4
   Any one of however many parts of speech regarded as the most fundamental or indispensable: The noun or the verb.
5. 5
   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.
6. 6
   The portion of a Laurent series that has negative exponents.
7. 7
   Used other than figuratively or idiomatically: see principal, part.