stem

stem (stamme): the main part of a word to which inflectional morphemes/suffixes may be added, viz. the base form of a verb,  the singular form of a noun, the positive form of adjectives and adverbs. It consists of a root, sometimes in combination with derivational affixes. In the word drivers, the stem is 'driver', and 's' is an inflectional suffix. The root is 'drive', and 'r' is a derivational suffix. The word unfaithful is a stem consisting of the root 'faith' and the two affixes 'un' and 'ful'.