this

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

• pronoun Used to refer to the person or thing present, nearby, or just mentioned.
• pronoun Used to refer to what is about to be said.
• pronoun Used to refer to the present event, action, or time.
• pronoun Used to indicate the nearer or the more immediate one.
• adjective Being just mentioned or present in space, time, or thought.
• adjective Being nearer or more immediate.
• adjective Being about to be stated or described.
• adjective Informal Used as a substitute for the indefinite article.
• adverb To this extent; so.

from The Century Dictionary.

• That is now present or at hand: a demonstrative adjective used to point out with particularity a person or thing that is present in place or in thought.
• Time just past or just at hand; the last or the next. The reference, whether to past or to future, is determined by the circumstances; this evening may mean either the evening now approaching, or next to come, or the evening now present, or the evening just past: as, it has occurred twice this year; I shall take care not to fail this (next) time. In this connection this is sometimes used for these, the sum being reckoned up, as it were, in a total.
• This person or thing.
• Something that has just preceded or has been mentioned or referred to.
• Emphatically, something that is to be immediately said or done: as, Let me tell you this: I shall lend you no more money.
• Elliptically, this person, place, state, time, position, circumstance, or the like: as, I shall leave this [place or town] to-morrow; this [state of affairs] is very sad; I shall abstain from wine from this [time] on; by this [time] we had arrived at the house.
• When opposed to that, this refers to the person or thing that is nearer, that to the person or thing that is more distant; so, with things that have just been expressed, this refers to the thing last mentioned (and therefore nearer in time to the speaker), and that to the thing first mentioned (as being more remote).
• For this; thus.

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

• preposition As a demonstrative pronoun, this denotes something that is present or near in place or time, or something just mentioned, or that is just about to be mentioned.
• preposition As an adjective, this has the same demonstrative force as the pronoun, but is followed by a noun.

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

• determiner The (thing) here (used in indicating something or someone nearby).
• determiner The known (thing) (used in indicating something or someone just mentioned).
• determiner The known (thing) (used in indicating something or someone about to be mentioned).
• determiner A known (thing) (used in first mentioning a person or thing that the speaker does not think is known to the audience). Compare with "a certain ...".
• adverb To the degree or extent indicated.
• pronoun The thing, item, etc. being indicated.
• noun philosophy Something being indicated that is here; one of these.
• interjection Internet slang Indicates the speaker's strong approval or agreement with the previous material.