precursor

详细解释:n. 先驱者, 前导, 前兆
【化】 前体; 前身; 产物母体
【医】 先质, 前体(产物母体)

英语解释：

名词 precursor:

a substance from which another substance is formed (especially by a metabolic reaction)a person who goes before or announces the coming of another同义词：forerunner