Writing Definitions

Definitions occur frequently in many types of scientific writing because it is often necessary to define certain operations, substances, objects or machines.

When you define anything the first part of the definition should be

general. The details should be left until later. In other words, the thing to be defined should be described first in terms of its general class, then in terms of particular properties, qualities, uses or origins. This could be expressed as:

T + G + (da + dc +…..+ dn)

where T equals the thing to be defined

where = equals "be"

where G equals a general class word

where da, db, etc. are the details which distinguish T from the other numbers of G.

(From Writing Scientific English)

Here is an example:

Epiphyte (T) is the plant (G) that grows upon another plant (da), but is

neither parasitic on it (db) nor rooted in the ground (dc).

i.e. T = G + da +db +dc

Although there are several possible ways of writing definitions in

English, there are two ways which are much commoner than the others:

Formula I

An (x/y) is a/an…general class word + wh- word…..

where x is a countable noun.

where y is an uncountable noun.

Here are some examples:

An x/y is a/an class word wh- word

A robot is a machine which can be programmed to performed specific mechanical functions in the manner of a man.

An expert is an AI computer which is designed system to represent human expertise in a particular domain.

Jet stream is a narrow wind which occurs in the current earth's atmosphere above the lower troposphere and flows towards the east at speeds of between 60-125 km/h.

Gunpowder is an explosive which consists of a mixture of potassium nitrate, sulphur, and charcoal.

Points to remember:
      1. "the" is not used with the subject because definitions are general statements.
      2. It is possible to use a reduced relative clause.
        1. In a reduced passive relative clause both the "wh-word" and "be" are omitted.

          2. e.g. An expert system is an AI computer program designed to represent human expertise in a particular domain.

        2. In a reduced active relative clause the "wh-word" is omitted and the verb is changed to a present participle (verb + ing).

e.g. Gunpowder is an explosive consisting of a mixture of potassium nitrate, sulphur, and charcoal.

Formula II

A/ an….general class word + wh- word…. is known as (an x/y).

is called

is termed

Here are some examples:

Class word wh- word is called /is known as a(n) x/y

is termed

A machine (which can be programmed to perform specific mechanical functions in the manner of a man is known as a robot.

An AI computer (which is) designed to represent human expertise in a particular domain is called an expert system.

An explosive which consists of a mixture of potassium , nitrate, sulphur, and charcoal is termed gun powder.

 

So far the general class word has always been followed by a wh- word or by a reduced relative clause. However, there may be a preposition before the wh- word. This happens when the subjects of the two parts of the definition sentence are not the same.

Here are some examples:

A direct current is an electric current in which the net flow of charge is in one direction only.

A thermometer is a device by means of which temperature is measured.

A drill is an instrument with which one can bore holes.

The temperature at which there is equilibrium between ice and water at standard atmospheric pressure is known as an ice point.

The temperature to which a substance must be heated before it will burn in air is termed ignition temperature.

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