同位语：句子中处于同等位置的两个名词性组成部分,其中一个成分是对另一个成分的解释或补充说明,如 My teacher,a retired professor,come to China two years ago.其中 My teacher 和 a retired professor 就是同为关系,后者补充说明前者.
同位语从句：上面所说的两个名词性成分中一个是从句,称作“同位语从句”.其特点为：
一个由 that/whether 或 when/where/how 引导的完整的句子
被该从句解释或补充说明的另一个名词性成分不在从句里扮演任何句子成分的角色.
比较：
定语从句：I have heard the story that you told me moments ago.（the story 既是主句的宾语,也是从句的间接宾语—— that you told me the story moments ago）
同位语从句：I have heard the story that an old lady was injured in the car accident.（the story 和从句 that an old lady was injured in the car accident 是同位关系,从句补充说明先行词 the story.这两个成分可以用一个系表结构来解释：The story is that an old lady was injured in the car accident.）