definition

The Definiendum: ex. a cat (what we are defining)
The Definien: the actual definition

There are five features that a good definition should have in order for it to be a good definition.

1. The definien must not be too broad. It can not exclude things that should be included. Ex: "A chair is a piece of furniture used to sit on." This definition is too broad because it allows things that shouldn't be included to be included such as a sofa bed, which would fit the definition.
2. The definien must not be too narrow, that is it should not include things that should be excluded. Ex: "A chair is a piece of furniture with four legs and is made of wood." This definition is too narrow because not all chairs have four legs, or are made of wood, and therefore it includes things that should be excluded.
3. The definien should state the most important attributes of the defieniendum; the "essence" of the object/thing. Ex: Defining a car in terms of metals used to create the car does not capture the purpose of a car, which is to drive/move around in, etc.
4. The defienien should not be circular. That is, the reader shouldn't have to know the definition of the word already in order to understand the definition. Ex: "A swimmer is a person that swims." - you would have to understand what swimming is in order to understand this definition.
5. The defienien should not be obscure, it should be understood by almost anyone. Ex: "A chair is an ontologically stable, space-time existent, constructed from natural or artificial materials and designed for human beings in non-reclining pose."

Def`i*ni"tion (?), n. [L. definitio: cf. F. d'efinition.]

1.

The act of defining; determination of the limits; as, the telescope accurate in definition.

2.

Act of ascertaining and explaining the signification; a description of a thing by its properties; an explanation of the meaning of a word or term; as, the definition of "circle;" the definition of "wit;" an exact definition; a loose definition.

Definition being nothing but making another understand by words what the term defined stands for. Locke.

3.

Description; sort.

Jer. Taylor.

4. Logic

An exact enunciation of the constituents which make up the logical essence.

5. Opt.

Distinctness or clearness, as of an image formed by an optical instrument; precision in detail.

Syn. -- Definition, Explanation, Description. A definition is designed to settle a thing in its compass and extent; an explanation is intended to remove some obscurity or misunderstanding, and is therefore more extended and minute; a description enters into striking particulars with a view to interest or impress by graphic effect. It is not therefore true, though often said, that description is only an extended definition. "Logicians distinguish definitions into essential and accidental. An essential definition states what are regarded as the constituent parts of the essence of that which is to be defined; and an accidental definition lays down what are regarded as circumstances belonging to it, viz., properties or accidents, such as causes, effects, etc."

Whately.

© Webster 1913.