Introduction:
In the English language, nouns are classified into two main categories: common nouns and proper nouns. Common nouns refer to general people, places, things, or ideas, while proper nouns specify unique individuals, specific locations, or brand names. In the case of the word "cat," it is categorized as a common noun.

Explanation:
A common noun is a general name for a person, place, thing, or idea, while a proper noun refers to a specific individual, location, or brand name. To understand why the word "cat" is classified as a common noun, let's explore the characteristics of both common and proper nouns.

Common Nouns:
1. Definition: Common nouns are general names given to people, places, things, or ideas.
2. Examples: Some examples of common nouns include dog, car, city, book, and cat.
3. Usage: Common nouns are not capitalized unless they begin a sentence.

Proper Nouns:
1. Definition: Proper nouns are specific names given to individuals, locations, or brand names.
2. Examples: Examples of proper nouns include John (individual), London (location), and Coca-Cola (brand name).
3. Capitalization: Proper nouns are capitalized regardless of their position in a sentence.

Explanation of "Cat" as a Common Noun:
Now, let's focus on the word "cat" and understand why it is considered a common noun.

1. General Name: The term "cat" is a general name given to a domesticated carnivorous mammal, commonly kept as a pet or used for pest control.
2. Non-Specific: When we say "cat," we are not referring to a particular cat, but rather any cat in general.
3. Not Capitalized: Common nouns like "cat" are not capitalized unless they begin a sentence. For instance, "The cat is sleeping."

Therefore, based on the characteristics and usage of common and proper nouns, we can conclude that the word "cat" is a common noun because it represents a general name for a domesticated carnivorous mammal without specifying any particular individual or location.