FORMATIVE.
1. In PHILOLOGY, a derivational AFFIX, especially one that determines part of speech or WORD class: -ness in darkness, forming a noun from an adjective.

2. In structural LINGUISTICS, a word-forming element: the prefix un- and suffix -ly in unkindly.

3. In generative GRAMMAR, a minimal unit of syntax: in The dancers performed gracefully, the formatives (joined by plus signs) are the + dance + er + s perform + ed grace + full + ly. Compare BASE, MORPHEME.

for·ma·tive / ˈfôrmətiv/ • adj. serving to form something, esp. having a profound and lasting influence on a person's development: his formative years. ∎  of or relating to a person's development: a formative assessment. ∎  Linguistics denoting or relating to any of the smallest meaningful units that are used to form words in a language, typically combining forms and inflections.• n. Linguistics a formative element.DERIVATIVES: for·ma·tive·ly adv.