the

from The American Heritage® Dictionary of the English Language, 5th Edition.

• definite article. Used before singular or plural nouns and noun phrases that denote particular, specified persons or things.
• definite article. Used before a noun, and generally stressed, to emphasize one of a group or type as the most outstanding or prominent.
• definite article. Used to indicate uniqueness.
• definite article. Used before nouns that designate natural phenomena or points of the compass.
• definite article. Used as the equivalent of a possessive adjective before names of some parts of the body.
• definite article. Used before a noun specifying a field of endeavor.
• definite article. Used before a proper name, as of a monument or ship.
• definite article. Used before the plural form of a numeral denoting a specific decade of a century or of a lifespan.
• definite article. Used before a singular noun indicating that the noun is generic.
• definite article. Used before an adjective extending it to signify a class and giving it the function of a noun.
• definite article. Used before an absolute adjective.
• definite article. Used before a present participle, signifying the action in the abstract.
• definite article. Used before a noun with the force of per.
• adverb To that extent; by that much. Used before a comparative.
• adverb Beyond any other.

from The Century Dictionary.

• A word used before nouns with a specifying or particularizing effect, opposed to the indefinite or generalizing force of a or an: as, the gods are careless of mankind; the sun in heaven; the day is fair; long live the king!
• A word used before a noun to indicate a species or genus: as, the song of the nightingale: used in generalization: as, the man that hath no music in himself.
• A word used with a title, or as part of a title: as, the Duke of Wellington; the Right Honorable the Earl of Derby; the Lord Brook; the Reverend John Smith.
• Indicating the most approved, most desirable, most conspicuous, or most important of its kind: as, Newport is the watering-place of the United States: in this use emphatic, and frequently italicized. The is often placed before a person's (especially a woman's) name, to indicate admiration or notoriety (a colloquial use): as, the Elssler.
• Before adjectives used substantively, denoting: An individual: as, she gazed long on the face of the dead.
• A class, or a number of individuals: as, the good die first; do not mix the new with the old.
• An abstract notion: as, the beautiful.
• Denoting that which is well known or famed: as, the prodigal son.
• Used distributively to denote any one separately: as, the fare is a dollar the round trip.
• Used in place of the possessive pronoun to denote a personal belonging: as, to hang the head and weep.
• Used to denote a particular day in relation to a given week, or to some other day of the same Week.
• Used before a participial infinitive, or gerund, followed by an object: the article is now omitted in this construction.
• Used before the relative which: now an archaism.
• [The is generally pronounced as if a syllable (unaccented) of the following word (a proclitic), and its vowel is accordingly obscured, before a consonant, into the neutral vowel-sound of her or but, very lightly sounded (quite like the French “mute e”); before a vowel, often in the same manner, but more usually with the short i sound of pin, only less distinct; when emphatic, as the long e of thee. In poetry, before a word beginning with a vowel-sound, the vowel of the generally may slide into that of the next word, and form with it one metrical syllable; metrically the e is accordingly often cut off in printing. The same so-called elision (synalephe) often took place in Middle English, the being written with the following noun as one word: as, themperour, the emperor.
• In Middle English manuscripts the was often written, as in Anglo-Saxon þe, with the character þ; in early print this character was represented by a form nearly like y, and later printers actually used y instead, þe, erroneously printed þ^e as if contracted, like þ^t for that, being printed ye or y^e but always pronounced, of course, the. Modern archaists often affect ye for the, and many pronounce it as it looks, “yē.”
• See thee.
• noun A Middle English form of thigh.
• Used to modify adjectives and adverbs in the comparative degree:
• Used without correlation, it signifies in any degree; in some degree: as, Are you well ? The better for seeing you.
• A Middle English form of though.

from the GNU version of the Collaborative International Dictionary of English.

• A word placed before nouns to limit or individualize their meaning.
• intransitive verb obsolete See thee.
• adverb By that; by how much; by so much; on that account; -- used before comparatives.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adverb With a comparative or more and a verb phrase, establishes a parallel with one or more other such comparatives.
• adverb this sense) With a comparative, and often with for it, indicates a result more like said comparative. This can be negated with none.