Definitions of formative:

• noun:   minimal language unit that has a syntactic (or morphological) function
• adjective:   beginning to develop
• adjective:   susceptible to alteration by development and experience

  Example: "Formative years"

• adjective:   relating to or characterized by formative effects or formation

  Example: "The formative arts"

• adjective:   forming or capable of forming or molding

  Example: "A formative influence"

• adjective:   capable of forming new cells and tissues

  Example: "A formative zone in developing bone"