定义文(definition)是一种对事物、词语和概念进行解释，以确定其本质内容和意义范围的篇章形式。定义文的根本目的在于释义，使词和概念含义清楚，所指明确，以消除歧义和混淆。因此，定义文是各种参考书、课本、论文及其他文献常使用的一种文体形式。
示例分析
上海道正认为，定义是一种常见的思维方式。为了统一理解和认识，人们总是对周围的事物和概念进行定义。由于事物和概念都以词语为介体，可以说，对前者的定义就是对词的定义。
一般来说，我们把定义词语或概念分为两个步骤：确定它们的意义范畴．然后说明这个词或概念与同一范畴的其他的词或概念的不同之处。例如：
1．词语或概念 意义范畴 不同之处
(term to be defined) (genus) (differentiae)
a. Psychology is the science of human and animal behavior.
b. Meteorolgy is the science of weather and climate.in which we imagine that is a defense
c. Identification we are not like another mechanism person. in which we retreat to is a defense early or primitive forms
d. ReGREss mechanism of behavior.
e. who is a physician or sure. A doctor is a person geon. on whom a university has
f. A doctor is aperson conferred one of its highest deGREes. licensed to practice any
g. A doctor is a person one of the healing arts.
who is a medicine man or
h. A doctor is a person witch doctor.如果说以上各句都是简单的定义形式的话，那么以下各
例便是定义的扩充形式：2. Democracy (term to be defined) refers primarily to a form of government (genus) by the many (difference) as opposed to government by one--gevernment by the people
(clarification of the difference) as opposed to government by a tyrant, or an absolute monarch.
在有些扩充的定义形式中，作者首先指出被定义的词的意义范畴，然后用描述的方法说明其特定的意义内涵，性质或特征，例如：3. A tiger, a member of the cat family, is native to Asia, usually weighs over 350 pounds, and has tawny and blackstriped fur.
一个完整的定义篇章都是对意义范畴和不同特征的不断扩充和充分阐述。这样的篇章往往具有以下结构模式：4. 首段 Introduction (including logical/sentence definition)
阐明意义范畴Expansion of the genus
阐述特定意义Expansion of the differentiae
尾段Conclusion(summary or restateinent)
上海道正认为作者往往按以占模式扩展定义文。下例就是用这种方法写成的一篇典型的定义文。作者是美国语言学家纳尔逊·弗朗西斯。文章的目的是定义"grammar"一词的三种不同含义。作者在文章首段指出，不同的人对"grammar"一词抱有不同的偏见，这是文章的引子以说明定义此词的必要性，此段的最后一句点明文章的主旨或中心思想。第二、三、和四段分别定义"grammar"的三种不同意义。第五段是全文的小结。为了使此文层次清楚，我们特用缩进式，以揭示句与句和段与段之间的主从和并列关系：
5. Three Meanings of Grammar
I
1 A curious paradox exists in regard to grammar.
2 On the one hand it is felt to be the dullest and driest of academic subjects, fit only for those in whose veins the red blood of life has long since turned to ink.
On the other, it is a subject upon which people who would scorn to be professional grammarians hold very dogmatic opinions, which they will defend with considerable emotion.
Much of this prejudice stems from the usual sources of prejudice--ignorance and confusion.
4 Even highly educated people seldom have a clear idea of what grammarians do,. and there is an unfortunate confusion about the meaning of the term "grammar" itself.
5 Hence it would be well to begin with definitions.
6 What do people mean when they use the word "grammar"?
7 Actually, the word is used to refer to three different things, and much of the emotional thinking about matters grammatical arises from confusion about these three different meanings.
此文结构十分严谨，全文严格按以下主旨句和正文各段的主题句展开，最后--句对全文进行总结：
8 The first thing we mean by "grammar" is "the set of formal patterns in which the words of a language are arranged in order to convey larger meanings. ' It is not necessary that we be able to discuss these patterns selfconsciously in order to be able to use them.
9 In fact, all speakers of a language above the age of five or six know how to use its complex form of organization with considerable skill ;in this sense of the word--call it "Grammar l" --they are thoroughly familiar with its grammar.II
The second meaning of "grammar" --call it "Grammar 2" mis "the branch of linguistic
science ,which is concerned with the description, analysis, and formularization of formal
:language patterns."9 Just as gravity was in full operation before Newton's apple fell, so grammar in the first sense was in full operation before anyone
formulated the first rule that began the history of grammar as a study.III
The third sense in which people use the word "grammar" is "linguistic etiquette."
This we may call "Grammar 3." 10 The word in this sense is often coupled with a derogatory adjective: we say that the expression "he aren't here" is "bad grammar."
10 What we mean is that such an expression is bad linguistic manners in certain circles.
11 From the point of view of "Grammar l" it is faultless; it conforms just as eompletely to the structural patterns of English as does "he isn't here. '
12 The trouble with it is like the trouble with Prince Hal in Shakespeare's
play--it is "bad," not in itself, but in the company it keeps.
IV
As has already been suggested, much confusion arises from mixing these meanings.
8 One hears a good deal of criticism of teachers of English couched in such terms as "they don't teach grammar any more."Criticism of this sort is based on the wholely unproved assumption that teaching Grammar 2 will increase the student's proficiency in Grammar l or improve his manners in Grarnmar 3.
10 Actually the form of Grammar 2 which is usually taught is a very inaccurate and misleading analysis of'the facts of Grammar 1; and it is therefore of highly questionable value in improving a person's ability to handle the structural patterns ofhis language.
11 It is hardly reasonable to expect that teaching a person some inaccurate
grammatical analysis will either improve the effectiveness of his assertions
or teach him what expressions are acceptable to use in a given context.
These, then, are the three meanings of "grammar": Grammar 1, a form of behavior; Grammar 2, a field of study, or science; and Grammar 3,a branch of etiquette.
by W. Nelson Francis (from Revolution in Grammar )
旨句 :The word [grammar is used to refer to three different things.
正文: The first [meaning of grammar] is "the set of formal
patterns in which the words of a language are arranged in order to convey larger meanings."
The second meaning of "grammar" ...is "the branch of linguistic -cience which is concerned with the de scription, analysis, and formularization of formal lan guage patterns."
The third [meaning of grammar] is "linguistic etiquette."
结论 : These, then, are the three meanings of "grammar" ...
a form of behavior, ...a field of study, or science, and...a branch of etiquette.
