Used as a relative pronoun in a clause that provides additional information about the antecedent: my house, which is small and old.
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.
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.
Any of the things, events, or people designated or implied; whichever: Choose which you like best.
A thing or circumstance that: He left early, which was wise.
What particular one or ones of a number of things or people: Which part of town do you mean?
Any one or any number of; whichever: Use which door you please.
Being the one or ones previously mentioned or implied: It started to rain, at which point we ran.