adj.
Function: pronoun
Date: before 12th century
1 : what one or ones out of a group ― used as an interrogative < which of those houses do you live in> < which of you want tea and which want lemonade> <he is swimming or canoeing, I don't know which >
2 : WHICHEVER <take which you like>
3 ― used as a function word to introduce a relative clause ― used in any grammatical relation except that of a possessive ― used especially in reference to animals, inanimate objects, groups, or ideas <the bonds which represent the debt ― G. B. Robinson> <the Samnite tribes, which settled south and southeast of Rome ― Ernst Pulgram> ― used freely in reference to persons as recently as the 17th century <our Father which art in heaven ― Mount 6:9(AV)>, and still occasionally so used but usually with some implication of emphasis on the function or role of the person rather than on the person as such <chiefly they wanted husbands, which they got easily ― Lynn White> ― used by speakers on all educational levels and by many reputable writers, though disapproved by some grammarians, in reference to an idea expressed by a word or group of words that is not necessarily a noun or noun phrase <he resigned that post, after which he engaged in ranching ― Current Biog. >
usage see 4 THAT