What is the only definite article?

"The" it is only definite article

it can be used for:

1. before a definite noun which has already been mentioned

2. when the singular noun represents a 'class'

3.before an adjective when it show a quality

4. before an adjective when it represents a 'class'

5. before the superlative form of am adjective

6. before a noun to show a unit measure

7. before the geographical proper nouns

8. before the cardinal directions and common nouns that are unique

9. before certain nouns