formative
1. for.ma.tive \'fo.r-m*t-iv\ aj 1a: giving or capable of giving form :
CONSTRUCTIVE {a ~ i nfluence} of an affix or other word element 1b: used
in word formation or inflection 2: capable of alteration by growth and
development; also : produci ng new cells and tissues 3: of, relating to, or
characterized by formative effects or formation { ~ years} - for.ma.tive.ly
av
2. formative n : the element in a word that serves to give the word
appropriate form and is not part of the base
Webster's English Dictionary