Etymology: Middle English, of what kind, which, from Old English hwilc; akin to Old High German wilīh of what kind, which, Old English hwā who, gelīk like — more at who, like
Date: before 12th century
1. being what one or ones out of a group — used as an interrogative <which tie should I wear> <kept a record of which employees took their vacations in July> 2. whichever <it will not fit, turn it which way you like> 3. — used as a function word to introduce a nonrestrictive relative clause and to modify a noun in that clause and to refer together with that noun to a word or word group in a preceding clause or to an entire preceding clause or sentence or longer unit of discourse <in German, which language might…have been the medium of transmission — Thomas Pyles> <that this city is a rebellious city… : for which cause was this city destroyed — Ezra 4:15 (Authorized Version)> II. 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 — Matthew 6:9(Authorized Version)>, 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 Biography> Usage: see that IV
New Collegiate Dictionary. 2001.