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Defining your topic

Introductory sections to most types of scientific writing begin with general statements and move to more specific statements. A text may begin with a formal or informal definition, or an extended definition.

For other types of writing you may wish to a) include a glossary of terms, or b) explain commonly held meanings, compared with your own understanding of the terms.

Formal definitions

In a formal sentence definition, the term being defined is first assigned to a class or group to which it belongs and then distinguished from other terms in that class. In most cases, the indefinite article "a" or "an" is used before both the term and the class. The second part of a definition contains a restrictive relative clause, indicated by the use of the relative pronouns "which" or "that". The relative pronoun "whose" is less commonly used.

Example:

A star is a celestial body which shines by itself and whose source of energy is nuclear fusion occurring in its core.

Note: in the above example "that" could be used instead of "which".

A sentence definition can be reduced by omitting the class to which the term belongs. In this way you avoid stating the obvious or repeating words unnecessarily.

Example:

A Beckman thermometer is a thermometer that measures small temperature differences.

The highlighted elements can be omitted. The reduced definition is:

A Beckman thermometer measures small temperature differences.

Informal definitions

Informal definitions usually occur more commonly in non-scientific writing. Here the definition does not follow the formal pattern of term + class + distinguishing feature(s). In an informal definition, the definition is often only implied: e.g. "Fosters is an open system organisation because it can influence, and be influenced by, its primary and secondary stakeholders."

Expanded definitions

An extended definition is used when a sentence definition is not sufficient. There are a number of ways to expand a definition. Here are the most common techniques.

  1. Further definition: words in the sentence definition are themselves defined. For example, the definition "A vaccine is a sterile liquid medium that contains an avirulent strain of a specific pathogen" is not completely clear unless "avirulent" and "pathogen" are also defined.
  2. Concrete examples: for example, if the word being defined is "pollution", the expanded definition may include examples of common forms of pollution.
  3. Description of parts or components
  4. Basic operating principles
  5. Purpose or method of use
  6. Cause and effect
  7. Word derivation: for example, if the term being defined is "n-type semiconductor", we need to be clear that 'n' means negative.
  8. Location and time
  9. Negative statement: for example, if the definition is "Thixotropy is the property of a liquid by which it has a lower viscosity at a higher rate of flow", the following sentence could state that this is not a property of most liquids

Click on the highlighted text to see the comments.

Below is an example of an expanded definition. Which of the techniques listed above are used? Tick all that apply.

An aneroid barometer is an instrument that depends on the changing volume of a container to indicate atmospheric pressure. It consists of an airtight box of thin flexible metal from which the air has been partially evacuated. One side of the evacuated box is attached to a spring. When atmospheric pressure increases, the box tends to collapse. When atmospheric pressure decreases, the sides of the box spring outward. This slight movement is magnified by a series of levers connected to an indicator, which shows the atmospheric pressure.

A variation of the aneroid barometer called the Bourdon gauge was invented by Eugene Bourdon, a French engineer. A flattened tube of metal is evacuated and bent into a circle. The tube tends to close up with greater pressure and open out with lesser pressure. This movement is transmitted to a dial as in the aneroid instrument. The Bourdon gauge is most suitable for measuring high pressure (for example, 2000 atmospheres).

Masters, P. 1986, Science, Medicine and Technology: English Grammar and Technical Writing, pp 27-29.

Check your answer

A number of the techniques are used; some can be seen to overlap.

An aneroid barometer is an instrument that depends on the changing volume of a container (4: basic operating principles) to indicate atmospheric pressure (5: purpose or method of use). It consists of an airtight box of thin flexible metal from which the air has been partially evacuated (3: description). One side of the evacuated box is attached to a spring (3). When atmospheric pressure increases, the box tends to collapse (4). When atmospheric pressure decreases, the sides of the box spring outward (4). This slight movement is magnified by a series of levers connected to an indicator, which shows the atmospheric pressure (5).

A variation of the aneroid barometer called the Bourdon gauge (2: concrete example) was invented by Eugene Bourdon, a French engineer. A flattened tube of metal is evacuated and bent into a circle (4). The tube tends to close up with greater pressure and open out with lesser pressure (5). This movement is transmitted to a dial as in the aneroid instrument (4). The Bourdon gauge is most suitable for measuring high pressure (for example, 2000 atmospheres) (5).

You will also, particularly in the non-science disciplines, find competing definitions being put forward, as in the following piece of student writing from Human Resource Management:

Rhetoric, as defined by Phillips (1997), is "the formal elements of an argument used to persuade an audience". Rhetoric has been the foundation of the study of verbal persuasion since the time of the ancient Greeks (Ong, 1997). In the organisational context, rhetoric can be an intentional tool to influence others (Morgan,1998), giving information about the organisation which assists employees to understand it in a certain way.

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