(of a clause) giving extra information about someone or something and not necessary in order to understand who or what is being referred to, or (of a pronoun) used in a clause like this

In the sentence "My younger sister, who lives in Boston, is a nurse", "who lives in Boston" is a non-defining relative clause. 在”My younger sister, who lives in Boston, is a nurse“这一句中，”who lives in Boston“ 是非限定性关系从句。

If the clause is non-defining and the antecedent non-human, you usually use which and put a comma before it, as in "He screwed the paper into a ball, which he threw in the bin." 如果这是一句非限定性（关系）从句且先行项为非人，你一般用“which"(引导从句）并在从句前加一个逗号，比如"He screwed the paper into a ball, which he threw in the bin."

In the first sentence "which" functions as a non-defining pronoun. 第一句中的“which“用作为非限定性（关系从句的）关系代词。