New Oxford American Dictionary describes "the" as an adjective
When I look at the definition given from the Mac OS X Dictionary, I read the following definition. (I have set American English as interface language, and the dictionary used is then the New Oxford American Dictionary.)
the /ði/ /ð(ə)/ /ði/ [called the definite article] adjective
- denoting one or more people or things already mentioned or assumed to be common knowledge: what's the matter? | call the doctor | the phone rang. Compare with a.
• used to refer to a person, place, or thing that is unique: the Queen | the Mona Lisa | the Nile.
• informal denoting a disease or affliction: I've got the flu.
• (with a unit of time) the present; the current: dish of the day | man of the moment.
• informal used instead of a possessive to refer to someone with whom the speaker or person addressed is associated: I'm meeting the boss | how's the family?
• used with a surname to refer to a family or married couple: the Johnsons were not wealthy.
• used before the surname of the chief of a Scottish or Irish clan: the O'Donoghue.
I have never thought of an article as adjective.
Is normally an article defined as adjective?
That is so wrong it makes my eyeballs bleed. Let's consider some syntactic tests, shall we?
Adjectives can be compared with -er and -est (or more and most):
A bigger house
The biggest house
! More the house
! Most the house
Adjectives can be placed in a predicate:
The house is big.
! House is the.
Adjectives can be coordinated with and:
Red and green house
! The and green house
Conclusion: the is not an adjective. The fact that it modifies a noun does not make it an adjective, because it has none of the grammatical or morphological properties of adjectives.
There might be a context where the definite article 'the' is used as an adjective. as in " not just any lawyer, the lawyer. This is to emphasize the illustrious nature of the person under consideration. I have seen it being used with proper nouns as well in the same context.