1.

denoting one or more people or things already mentioned or assumed to be common knowledge.

"what's the matter?"

used to refer to a person, place, or thing that is unique.

"the Queen"

INFORMAL•ARCHAIC

denoting a disease or affliction.

"I've got the flu"

(with a unit of time) the present; the current.

"dish of the day"

INFORMAL

used instead of a possessive to refer to someone with whom the speaker or person addressed is associated.

"I'm meeting the boss"

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"

2.

used to point forward to a following qualifying or defining clause or phrase.

"the fuss that he made of her"

(chiefly with rulers and family members with the same name) used after a name to qualify it.

"George the Sixth"

These are the first 2 definitions and the examples associated with the definitions of the word "the" on google's dictionary.