Principal part

In morphology, the principal parts of a paradigm are a minimal subset of the paradigm from which the formation of all the other members of the paradigm can be deduced.

Example
In Latin grammar, the principal parts of verbs are often taken to be the first person singular present indicative active, the infinitive, the 1st person singular perfect indicative active, and the perfect passive participle:

laudo   laudare    laudavi     laudatum moneo   monere     monui       monitum duco    ducere     duxi        ductum capio   capere     cepi        captum audio   audire     audivi      auditum

Reference

 * Finkel, Raphael & Stump, Gregory. 2007. Principal parts and morphological typology. Morphology 17: 39-75.

Other languages
German Stammform