which

Từ điển Oxford Advanced Learner 8th

which BrE [wɪtʃ] NAmE [wɪtʃ] pronoun, determiner
1. used in questions to ask sb to be exact about one or more people or things from a limited number
• Which is better exercise— swimming or tennis?
• Which of the applicants has got the job?
•Which of the patients have recovered?
•Which way is the wind blowing?
compare ↑what
2. used to be exact about the thing or things that you mean
• Houses which overlook the lake cost more.
• It was a crisis for which she was totally unprepared.  That can be used instead of which in this meaning, but it is not used immediately after a preposition
•It was a crisis that she was totally unprepared for.
3. used to give more information about sth
• His best movie, which won several awards, was about the life of Gandhi.
• Your claim ought to succeed, in which case the damages will be substantial.  That cannot be used instead of which in this meaning.
Idiom: ↑which is which

Word Origin:
[which] Old English hwilc, from the Germanic bases of ↑who and ↑alike.