定语从句是用来修饰 名词或代词的从句。被修饰的词称为先行词，定语从句通常放在先行词之后，并通过关系词（如that, who, which, whom, whose等）连接到主句。定语从句在句子中充当定语成分，提供关于先行词的额外信息，如大小、颜色、数量、所属等。
例如：
1. The book that I borrowed from the library is very interesting.
2. The woman who is sitting next to me is a doctor.
3. The house, which has three bedrooms, is very large.
在这些例子中，定语从句分别修饰了名词"book"和"woman"，并提供了关于它们的额外信息。