The elusiveness of success means everyone defines it differently.

elusiveness 是形容词 elusive 的名词形式，由后缀 -ness 构成。elusive 来自动词 elude（逃避、躲避），意思是"难以捉摸的、难以理解的"。elusiveness 表示"难以捉摸的特性"。