formative

adj

1. Capable of forming something.

   “[I]ts thought, that, in the Seed are alvvaies potentially ſeuerall indiuiduating Qualities deriu'd from diuers of the neere Anceſtors, vvhich by the formatiue povver of the Parents may be expreſt in the Children, vvith reſpectiue habitude to either Sex; […]”

   “Hybrids, […] have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the formative organs themselves are perfect in structure, as far as the microscope reveals.”

2. Capable of forming something.
3. Capable of forming something.
4. Of or pertaining to the formation and subsequent growth of something.

   “My formative years were spent in an inner city.”

5. Of a form of assessment: used to guide learning rather than to quantify educational outcomes.

noun

1. A thing which causes formation to occur.

   “[T]his museum of the state of flux [Newmarket, Suffolk] has a climate unrivalled for the production of the British temperament. Not without a due proportion of that essential formative of character, east wind, it has at once the hottest sun, the coldest blizzards, the wettest rain, of any place of its size in 'the three kingdoms.'”

2. A language unit, typically a morph, that has a morphological function (that is, forming a word from a root or another word).
3. Synonym of derivative (“a word that derives from another one”).