which

American Heritage® Dictionary of the English Language, Fourth Edition

1. pro. What particular one or ones: Which of these is yours?
2. pro. The one or ones previously mentioned or implied, specifically:
3. pro. Used as a relative pronoun in a clause that provides additional information about the antecedent: my house, which is small and old.
4. pro. Used as a relative pronoun preceded by that or a preposition in a clause that defines or restricts the antecedent: that which he needed; the subject on which she spoke.
5. pro. Used instead of that as a relative pronoun in a clause that defines or restricts the antecedent: The movie which was shown later was better.
6. pro. Any of the things, events, or people designated or implied; whichever: Choose which you like best.
7. pro. A thing or circumstance that: He left early, which was wise.
8. adj. What particular one or ones of a number of things or people: Which part of town do you mean?
9. adj. Any one or any number of; whichever: Use which door you please.
10. adj. Being the one or ones previously mentioned or implied: It started to rain, at which point we ran.

Century Dictionary and Cyclopedia

1. A. interrog. What one of a certain implied number or set? indicating a general knowledge of a certain group of individuals, and seeking for a selection of one or more from that number: thus, which do you want? implying a limitation which is absent from the question what do you want?
2. B. rel. As a simple relative pronoun: Who or whom.
3. Used with, reference to things, and to creatures not persons: the antecedent may also be a phrase or a clause: as, the rain washed away the track, which delayed the train.
4. As a compound relative pronoun, having the value of both antecedent and relative: as, you can determine which is better (that is, you can determine that, or the one, which is better).
5. Which is used adjectively:
6. With the sense of ‘what sort of.’
7. As indicating one of a number of known or specified things: as, be careful which way you turn.
8. [Which was formerly used as a clause-connective, along with a personal pronoun which took its place as subject or object, and rendered it redundant save as in its relative value: as, which … he = who; which … his = whose.
9. A relic of this construction survives in the vulgar use of which as a general introductory word.
10. Which was formerly often followed by that or as, having the effect of giving emphasis or definiteness.
11. Redundant for which.
12. n. A chest.
13. n. Specifically, a movable wagon-box.

Wiktionary

1. What, of those mentioned or implied (used interrogatively).
2. interrogative What one or ones (of those mentioned or implied).
3. relative The one or ones that.
4. relative the one mentioned
5. now dialectal Used of people (now generally who, whom or that).
6. pro. relative Who; whom; what (of those mentioned or implied)
7. n. An occurrence of the word which.

GNU Webster's 1913

1. pro. obsolete Of what sort or kind; what; what a; who.
2. pro. A interrogative pronoun, used both substantively and adjectively, and in direct and indirect questions, to ask for, or refer to, an individual person or thing among several of a class; See the Note under What, pron., 1.
3. pro. A relative pronoun, used esp. in referring to an antecedent noun or clause, but sometimes with reference to what is specified or implied in a sentence, or to a following noun or clause (generally involving a reference, however, to something which has preceded). It is used in all numbers and genders, and was formerly used of persons.
4. pro. A compound relative or indefinite pronoun, standing for any one which, whichever, that which, those which, the … which, and the like.