先行词是被定语从句所修饰的名词、代词，在主句中充当一定的部分。
被定语从句修饰的名词、代词称为先行词。我们可以这样认为，之所以称它为先行词，是因为修饰它的定语从句总是放在它之后，先行词总是出现在定语从句之前，而不同于普通形容词修饰名词时，形容词位于名词之前。
限定性
举例：where,which,that,who,whose
大多数定语从句对所修饰词的意思加以限制，表示“...的人（或东西）”，称为限定性定语从句。
限定性定语从句，用来修饰和限定先行词，与先行词间的关系非常密切，它所修饰的先行词代表一个（些）或一类特写的人或物，从句用来说明先行词的性质、身份、特征等状况，如果去掉，则意思含混不清。
A doctor is a person who looks after people’s health. 医生就是照顾病人的人。
种类：常见的关系代词、常见的关系副词。
引导定语从句的词被称为关系词。关系词根据其在定语从句中的功能可分为关系代词和关系副词。关系代词在定语从句中作主语、宾语、表语、定语等，关系副词在定语从句中作状语。含有定语从句的句子一般分为两层：即主句层（第一层），定语从句层（第二层）。
例句
例句：This is the book which I am looking for.
它是我正在寻找的书。
其中的“the book”是先行词，“which”是引导词。
例句：He is Mr. Robertson who comes from England.
他是来自英国的罗伯森先生。
其中的“Mr. Robertson”是先行词，“who”是引导词。
例句：Was that a singer that sang Xi Yangyang？
那个是唱喜洋洋的歌手吗？
其中的“a singer”是先行词，“that”是引导词。
例句：The finest eloquence is that which gets things done.
最好的雄辩是把事情做成。
其中that是先行词，which引导状语从句。
本回答被网友采纳被定语从句修饰的名词、代词称为先行词。
我们可以这样认为, 之所以称它为先行词, 是因为修饰它的定语从句总是放在它之后, 先行词总是出现在定语从句之前, 而不同于普通形容词修饰名词时, 形容词位于名词之前。
This is the book which I am looking for.中的book是先行词,which是引导词.
He is Mr. Robertson who comes from England. 中的Mr. Robertson.是先行词,who是引导词.
简单的说，就是和修饰宾语的词或从句，词就是先行词
1、先行词指人时，引导词可用Who,Whom,及that,其中当引导词在句中作主语时用Who或that；当其充当宾语时用whom或that，在非正式宾语中也可用who，也可省略。
a That boy who/that broke the window is called Tom.
b The person to whom you just talked is Mr Li.
b 句可改为：The person(that/whom/who)you just talked to is Mr Li.
2、先行词指物时，引导词用that或which
a.They planted the trees which didn’t need much water.
b .The fish (which)we bought were not fresh.
3、先行词指时间时，引导词用When/on which或in which
a.I still remember the day when/on which I first came to Bei jing.
b.I'll never forget the year when/ on which we worked on the farm.
4、先行词指地点时，引导词用Where或in which
This is the house where/in which we lived last year.
或:This is the house that/which we lived in last year.
5、whose在定语从句中作定语时，可用来指人，也可以用来指物，如:
a.This is the boy whose mother died a month ago.
b.This is the house whose windows were broken.此句可改成：
This is the house the windoues of which were broken.
6、先行词表示原因时，引导词用why,如:
The reason why he was late for school was that he had been knocked down by a bike.本回答被提问者采纳
the book which is on the desk
the book 是先行词； which 是关系代词。
我想你大概要问的是 关系词。下面给你作个归纳。
关系词分为关系代词和关系副词。
先说关系代词。
修饰物的有: that which whose
修饰人的有 that who whom whose
还有特殊的 as, but(该词估计你上了大学英语专业会详细学到）
常见的关系副词有;
where when why that(特殊情况下也作副词，如用在 way 的后面）。
最后，祝你进步！