which 
(wĭch, hwĭch)
(wĭch, hwĭch)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 the Appendix of Indo-European roots.]
Usage Note: The relative pronoun which can sometimes refer to a clause or sentence, as opposed to a noun phrase: She ignored him, which proved to be unwise. They swept the council elections, which could never have happened under the old rules. More than 80 percent of the Usage Panel approved both of these examples in our 2009 survey. Sometimes which clauses of this sort are presented as separate sentences. These are technically sentence fragments, and they often pack a rhetorical punch: "I was caught for a week on the Siachen Glacier, in a giant blizzard. There is no harsher place on this earth; it belongs to no one. Which won't keep people from squabbling over it someday" (Andrea Barrett). While this example is perfectly acceptable, writers who want to avoid this use of which and adhere to the traditional rules can usually substitute this for it at the start of a new sentence, though often at the loss of some dramatic flair. · Note that which clauses that modify whole sentences can sometimes create ambiguities. The sentence It emerged that Martha made the complaint, which surprised everybody may mean either that the complaint itself was surprising or that it was surprising that Martha made it. This ambiguity may be avoided by using other constructions such as It emerged that Martha made the complaint, a revelation that surprised everybody. Remember that which is used in this way only when the clause or sentence it refers to precedes it. When the clause or sentence follows, writers must use what, particularly in formal style: Still, he has not said he will withdraw, which is more surprising. Still, what is more surprising, he has not said he will withdraw. See Usage Notes at that, what, whose.