1. Intension and Extension
Prof. Dr. Walid Amer

2. Extension and Intension describe two ways of indicating the meaning of a word or name.
“Intension” indicates the internal content of a term or concept that constitutes its formal definition. (sense)
“Extension” indicates its range of applicability by naming the particular objects that it denotes. (denotation)

3. Extension
The extension of a lexeme is the set of entities which it denotes.
For example:
The extension of the word "chair" includes every chair that is (or ever has been or ever will be) in the world.

4. All the things that can be denoted by the noun lake are the extension of that lexeme .
The lexeme Lake Ontario has a single item in its extension, and the Dead Sea Scrolls has a single collection of items as its extension.

5. Intension
The intension of a general term, on the other hand, is the set of features which are shared by everything to which it applies.
For example :
The intension of the word "chair" is (something like) "a piece of furniture designed to be sat upon by one person at a time."

6. In other words, the intension of any lexeme is the set of properties shared by all members of the extension.
Thus everything that is denoted by lake must be a body of land surrounded by water.

7. Clearly, these two kinds of meaning are closely interrelated
Clearly, these two kinds of meaning are closely interrelated. We usually suppose that the intension of a concept or term determines its extension, that we decide whether or not each newly-encountered piece of furniture belongs among the chairs by seeing whether or not it has the relevant features. Thus, as the intension of a general term increases, by specifying with greater detail those features that a thing must have in order for it to apply, the term's extension tends to decrease, since fewer items now qualify for its application.

8. the intension of “ship” as a substantive is “vehicle for conveyance on water,” whereas its extension embraces such things as cargo ships, passenger ships, battleships, and sailing ships. The distinction between intension and extension is not the same as that between connotation and denotation.

9. Extension has to do with reference, but reference is not all of meaning : the lexeme violin and fiddle have the extension.
Extension can change while intension remains the same.
The extension of the referring expression the capital of Australia is a single item, the city of Canberra. The intension of the same term is city in which the national government of Australia is located.

10. Note :
The extension of the hyponym is included in the extension of the superordinate ( all collies are dogs), but the intension of the superordinate is included in the intension of the hyponym ( the characteristic of being a dog is part of the characteristic of being a collie ).

11. Prototype
A prototype is an object or referent that is considered typical of the whole set.
A prototype is a cognitive reference point, i.e the proto-image of all representatives of the meaning of a word or of a category. Thus, a robin or a sparrow can be regarded as a prototype or a "good example" of the category bird, whereas a penguin or an ostrich is a rather "bad example" of this category.

12. Accordingly, the members of a category can be graded according to their typicality. A "good" example is only rated as such by virtue of its features. Defining a prototype as the bundle of typical features of a category, we can thus imagine birds as 'creatures that are covered with feathers, have two wings and two legs, and the majority of which can fly'. Therefore, a penguin is a less "good" bird, as it lacks some of the typical features, such as the ability to fly. Features themselves can also be more or less typical, for example 'twittering' is less typical and specific to birds than 'flying'.