科技写作常常需要给一些术语下定义，常用的下定义的方法有三种：

1.给出一个同义词；

2.写一句话；

3.写一个段落。

一个词或一个句子固然可以对一个术语进行简单定义，但当该术语十分深奥难懂或者需要读者对其进行更详尽的了解时，就必须使用第三种方法了。

通过段落进行定义时，一般先指出“某某就是……”（简单定义），然后再在此基础上进行扩展，通过比较、对照和解释说明等方法，说明被定义项的作用、功能和特点，介绍有关对象的背景、起源、分支和流派或者所含的各个组成部分等等，使读者能够充分、透彻地了解该事物。

简单定义常用的表达方法有：

A is B+定语从句

A is B+介词短语/分词短语/不定式短语（作定语）

A deals with...

A refers to.../means...

A is used in...to denote...(www.daowen.com)

A is defined as/known as B

例1：

Hydrogeology is a science dealing with the properties，distribution，and circulation of water on the surface of the land，in the soil and underlying rocks，and in the atmosphere.The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passed，beginning as atmospheric water vapor，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation and transpiration.

在上段中，作者为了介绍水文循环，首先通过一句话给水文地质学下定义，然后指出水文循环是水文地质学的一个主要课题，并通过描述其循环过程说明它是水进行的一个完整周期循环。

例2：

Photovoltaics is a technology that typically transforms sunlight into electricity.Radiation from the visible part of the spectrum is，after all，abundant，nonpolluting and free.But photovoltaics can also provide useful amounts of electricity from infrared radiation—that is，radiant heat generated by a source of energy such as fuel oil.

在该段中，作者通过对比“photovoltaics”和“Radiation from the visible part of the spectrum”，使读者更明确地了解到光电技术的优势。

例3：

What is a breakwater？A breakwater is a physical device that blocks or damps wave action.Breakwaters are used to provide an alley of safe passage for ships and boats moving，across the surf zone，from the harbor to the open water.

该段首先通过设问对“breakwater”进行简单定义，然后又对其用途进行了介绍。