在复合句中作定语，修饰主句或主句中某一名词或代词的从句叫定语从句。定语从句由关系词引导。限制性定语从句表现为在意义上是先行词不可缺少的定语，若省略，主句的内容就不完整或失去意义；它和主句的关系十分密切，不可用逗号分开。
先行词是被定语从句修饰的名词或代词。关系词指代先行词，在从句中作一定的句子成分。引导定语从句的关系代词有which，that，who，whom，whose，as等。
[观察例句]
1．Do you know the man that/who is waiting outside.
2．He showed me the photos (that/which) he took on his trip to Xi'an.
3．Simon is the poor man whose car has been stolen.
4．I like the way (that/in which) she smiles.
5．The trees that/which are behind the office building have lost their leaves.
6．Then another big quake which was almost as strong as the first one shook Tangshan.
7．The army organized teams to dig out those who were trapped and to bury the dead.
8．Workers built shelters for survivors whose homes had been destroyed.
