ADVERB:

To the same extent or degree; equally: The child sang as sweetly as a nightingale.
For instance: large carnivores, as the bear or lion.
When taken into consideration in a specified relation or form: this definition as distinguished from the second one.
CONJUNCTION:

To the same degree or quantity that. Often used as a correlative after so or as: You are as sweet as sugar. The situation is not so bad as you suggest.
In the same manner or way that: Think as I think.
At the same time that; while: slipped on the ice as I ran home.
For the reason that; because: went to bed early, as I was exhausted.
With the result that: He was so foolish as to lie.
Though: Great as the author was, he proved a bad model. Ridiculous as it seems, the tale is true.
In accordance with which or with the way in which: The hotel is quite comfortable as such establishments go. The sun is hot, as everyone knows.
Informal That: I don't know as I can answer your question.
PRONOUN:

That; which; who. Used after same or such: I received the same grade as you did.
Chiefly Upper Southern U.S. Who, whom, which, or that: Those as want to can come with me.
PREPOSITION:

In the role, capacity, or function of: acting as a mediator.
In a manner similar to; the same as: On this issue they thought as one.

IDIOMS:
as is Informal
Just the way it is, with no changes or modifications: bought the samovar as is from an antique dealer.
as it were
In a manner of speaking; as if such were so.

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ETYMOLOGY:
Middle English, from Old English ealsw ; see also
Usage Note:
A traditional usage rule draws a distinction between comparisons using as . . . as and comparisons using so . . . as. The rule states the so . . . as construction is required in negative sentences (as in Shakespeare's "'tis not so deep as a well"), in questions (as in Is it so bad as she says?), and in certain if- clauses (as in If it is so bad as you say, you ought to leave). But this so . . . as construction is becoming increasingly rare in American English, and the use of as . . . as is now entirely acceptable in all contexts.·In a comparison involving both as . . . as and than, the second as should be retained in written style. One writes He is as smart as, or smarter than, his brother, not He is as smart or smarter than his brother, which is considered unacceptable in formal style.·In many dialects, people use as in place of that in sentences like We are not sure as we want to go or It's not certain as he left. This construction is not sufficiently well established to be used in writing.·As should be preceded by a comma when it expresses a causal relation, as in She won't be coming, as we didn't invite her. When as expresses a time relation, it is not preceded by a comma: She was finishing the painting as I walked into the room. When beginning a sentence with a clause that starts with as, one should take care that it is clear whether as is used to mean "because" or "at the same time that." The sentence As they were leaving, I walked to the door may mean either "I walked to the door because they were leaving" or "I walked to the door at the same time that they were leaving."·As is sometimes used superfluously to introduce the complements of verbs like consider, deem, and account, as in They considered it as one of the landmark decisions of the civil rights movement. The measure was deemed as unnecessary. This usage may have arisen by analogy to regard and esteem, with which as is standardly used in this way: We regarded her as the best writer among us. But the use of as with verbs like consider is not sufficiently well established to be acceptable in writing. See Usage Notes at because, equal, like 2, so 1, than.
Regional Note:
American dialects often vary from Standard English in the form and usage of relative pronouns. Where Standard English has three relative pronounswho, which, and thatregional dialects, particularly those of the South and Midlands, allow as and what as relative pronouns: "Them as thinks they can whup me jest come ahead" (Publication of the American Dialect Society). The car what hit him never stopped.