pronoun

1. what one (or ones) of the number of persons, things, or events mentioned or implied? which of the men answered? which do you want?
2. the one (or ones) that he knows which he wants
3. that: used as a relative referring to the thing, group, or event specified in the antecedent word, phrase, or clause: which can be used in a restrictive clause [the war which had just ended, the class to which he spoke], in a restrictive clause preceded by the pronoun that[he sacrificed that which he valued most], in a nonrestrictive clause [my car, which is not running; my family, in which she found a warm welcome], or, archaically, of a person [Our Father, which art in heaven]
4. either, or any, of the persons, things, or events previously mentioned or implied; whichever take which you prefer
5. a thing or fact that you are late—which reminds me, where were you yesterday?

Etymology: ME whiche < OE hwylc, hwelc, for *hwa-lic, lit., who like (akin to Goth hwileiks, OHG hwelīh, Ger welch): see who & -ly

adjective

1. what one or ones (of the number mentioned or implied) which man (or men) answered? which books did he choose?
2. whatever; no matter what try which method he pleased, he could not succeed
3. being the one just mentioned he is very old, which fact is important