English grammar rule The Superlative
Unique nouns are indicated by using the definite article the. Nouns become unique when modified with a superlative (best, worst) or an ordinal number (tenth, hundredth). There are other adjectives that can also make a noun unique depending on context (“The white house” when others around it are blue.)
Examples of 'The Superlative' rule
Incorrect They had a reception for an champion runner. Correct They had a reception for the champion runner. Incorrect The new teacher will start on a second day of the school year. Correct The new teacher will start on the second day of the school year. Incorrect That was a final straw for the principal. Correct That was the final straw for the principal.