/ ˈfɔːmətɪv; NAmE ˈfɔːrm-/ adjective
[ only before noun ] having an important and lasting influence on the development of sth or of sb's character :
the formative years of childhood
••
WORD ORIGIN
late 15th cent.: from Old French formatif , -ive or medieval Latin formativus , from Latin formare to form, based on Latin forma a mould or form.