定语从句（Attributive Clauses），在句中作定语，其作用是修饰主句的某个名词性成分，相当于形容词，所以又称为形容词性从句。通常定语从句皆置于它所修饰的名词或代词之后，被修饰的名词或代词叫做先行词（antecedent），而引导定语从句的词称为关联词。

雅思培训中心分析认为，关联词常有3个作用：连接作用，引导定语从句；代替先行词；在定语从句中担当一个成分。关联词为关系代词和关系副词。关系代词在定语从句中可以用作主语、宾语、定语等；关系副词在定语从句中只用作状语。如：

★ One factor which may help ensure that some endangered languages do not die out completely is people’s increasing appreciation of their cultural identity.（which may help ensure that some endangered languages do not die out completely是关系代词which引导的定语从句，用以修饰which的先行词factor, which在从句中用作主语）

★ And even in regions where new projects seem warranted, we must find ways to meet demands with fewer resources, respecting ecological criteria and to a smaller budget.（where new projects seem warranted是关系副词where引导的定语从句，用以修饰where