The (determiner)

1. Used to indicate one or more people or things already mentioned or assumed to be common knowledge.
2. Used to indicate the one or ones already mentioned or assumed to be common knowledge.

Origin:

Old english, of germanic origin; related to dutch de and german der.

Examples:

1. I saw the man in the park.
2. The book is on the table.
3. The dog barked at the mailman.
4. The teacher assigned the homework.
5. The weather is nice today.