which

Từ điển American Heritage Dictionary 4th

which (hwĭch, wĭch)pron.
1. What particular one or ones:
Which of these is yours?
2. The one or ones previously mentioned or implied, specifically:
a. Used as a relative pronoun in a clause that provides additional information about the antecedent:
my house, which is small and old.
b. Used as a relative pronoun preceded by that or a preposition in a clause that defines or restricts the antecedent:
that which he needed; the subject on which she spoke.
c. Used instead of that as a relative pronoun in a clause that defines or restricts the antecedent:
The movie which was shown later was better.
3. Any of the things, events, or people designated or implied; whichever:
Choose which you like best.
4. A thing or circumstance that:
He left early, which was wise.adj.
1. What particular one or ones of a number of things or people:
Which part of town do you mean?
2. Any one or any number of; whichever:
Use which door you please.
3. Being the one or ones previously mentioned or implied:
It started to rain, at which point we ran. [Middle English, from Old English hwilc. See kʷo- in Indo-European Roots.] 
Usage Note: The relative pronoun which is sometimes used to refer to an entire sentence or clause, rather than a noun or noun phrase, as in She ignored him, which proved to be unwise. They swept the council elections, which could never have happened under the old rules. While these examples are unexceptionable, using which in this way sometimes produces an ambiguous sentence. Thus It emerged that Edna made the complaint, which surprised everybody leaves unclear whether it was surprising that a complaint was made or that Edna made it. The ambiguity can be avoided with paraphrases such as It emerged that the complaint was made by Edna, a revelation that surprised everybody.·Which may be used to refer to an entire sentence or clause only when it is preceded by that sentence or clause. When the referent follows, what should be used, particularly in formal style: Still, he has not said he will withdraw, which is more surprising but Still, what (not which) is more surprising, he has not said he will withdraw. See Usage Note at that. See Usage Note at what. See Usage Note at whose.