A definition states the meaning of a term. Since meaning of an expression is explained by its definition, definitions provide a useful method of preventing or eliminating differences in the use of languages.
In other words, definitions are essential for preventing ambiguity. We observe earlier that verbal disputes often arise due to use of ambiguous and excessively vague words are phrases. Such disputes can be avoided if we agree on the definition of the key terms in our arguments. Definitions thus help us in correcting mistakes in our reasoning by eliminating ambiguity.
The symbol we seek to define is called the definiendum. The symbol or group of symbols used to state the meaning of the term defined is called the definiens. In other words, the term sought to be defined is the definiendum and the expression supplying the definition is the definiens. Consider the definition of ‘triangle’.
A triangle is a plane figure enclosed by three straight lines.
In this definition the term ‘triangle’ is the definiendum and ‘a plane figure enclosed by three straight lines’ is the definiens. The definiendum and the definiens are said to have the same meaning. Both are claimed to be synonymous expressions and therefore one can be substitute for the other without any loss of meaning.
A definition is either reportive or stipulative. When a definition states the meaning of a term as it is used it is called reportive definition. When it states the meaning of a term as it is to be used it is called stipulative definition.
A reportive definition of a term reports the way in which the term is already used. It tells us how a term is actually used by the native speakers of the language. Such a definition reports the conventional meaning of a term. It reports the long established usage of a term.
Reportive definitions can be true or false, depending upon the accuracy with which it captures the actual usage of the term. Reportive definitions help to eliminate ambiguity. Reportive definitions also help us to increase our vocabulary by learning the meaning of new symbols by the help of symbols we already understand.
Most dictionary definitions are examples of reportive definitions. ‘A triangle is a plane figure enclosed by three straight lines’ is a reportive definition. So also the definition of ‘unicorn’ as ‘a horse-like animal having a single straight horn projecting from its forehead’ is a reportive definition.
A stipulative definition assigns a new meaning to a term. By stipulative definitions new terms are introduced into a language or old terms are made precise by deliberately assigning new meaning for the terms.
A stipulative definition creates a new usage that did not exist prior to the stipulation. Stipulative definitions have the form “Let us use the word’…’ to mean’…’.
Stipulative definitions are neither true nor false. One can however judge a stipulative definition to be useful or cumbersome, clear or obscure, but not as true or false. Since the purpose is to introduce a term by assigning a new meaning, no existing convention or usage can be cited for its correctness or incorrectness.
A stipulative definition is a proposal or a decision to use a term in a certain sense. When a term is first introduced into a language it gets its meaning by stipulation. For example, when spacecrafts were made to fly to outer space and persons were specially trained to fly these spacecrafts, there was a need to introduce a technical term to refer to such persons. So the term ‘astronaut’ was coined to mean ‘a person who has been trained to travel in a spacecraft’. When the word was first introduced to mean this it was an instance of stipulative definition.
Now, however, the term has already gained wide currency and this definition can be cited as an instance of reportive definition.