—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 (King James Version)
—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 (King James 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
that, which, or who?: Usage Guide
In current usage that refers to persons or things, which chiefly to things and rarely to subhuman entities, who chiefly to persons and sometimes to animals. The notion that that should not be used to refer to persons is without foundation; such use is entirely standard. Because that has no genitive form or construction, of which or whose must be substituted for it in contexts that call for the genitive.
That vs. Which: Usage Guide
Although some handbooks say otherwise, that and which are both regularly used to introduce restrictive clauses in edited prose. Which is also used to introduce nonrestrictive clauses. That was formerly used to introduce nonrestrictive clauses; such use is virtually nonexistent in present-day edited prose, though it may occasionally be found in poetry.
—used as an introductory particle (see particlesense 4) before a word or phrase that is a reaction to or commentary on the previous clause
I have a very big reputation in Vancouver for being a sore loser, which, fair enough.—Ilona Verley
This morning we have the monthly jobs report, which who knows if it will meet or beat expectations.—Todd 'Bubba' Horwitz
Scientists have discovered the bones of a three-foot-tall, 15-pound parrot that lived some 16 million years ago. The remains had initially been misidentified as those of an "enormous, possibly human-eating eagle," which … yikes.—Oliver Roeder
Word History
Etymology
Adjective
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
First Known Use
Adjective
before the 12th century, in the meaning defined at sense 1
Pronoun
before the 12th century, in the meaning defined at sense 1
that, which, or who?: Usage Guide 
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