定语从句是英语语法现象，在复合句中 修饰名词或代词的从句叫定语从句，被修饰的名词或代词叫先行词，有关系代词who, whom, whose, which, that等和关系副词where, when, why等，关系代词和关系副词在定语从句中担任句子成份。