Formative is a word that describes something that made you who you are. You might call your adolescence your formative years because that time period had such a strong influence on the rest of your life.
The word form means “to shape.” Something that is formative is capable of shaping or molding something or someone. A formative experience is one that strongly influenced you, like the first time you went to the beach and decided, from that day, that you wanted to become a world-class surfer. Formative can also describe an organism that is able to form new cells. In linguistics, the noun formative is a small language unit that can help shape or form a word.