which Definition

which
adj, pronoun

1. Used in questions, indirect questions and statements: to identify or specify a thing or person, usually from a known set or group

Example: can't decide which book is better
Example: Which did you choose?
2. Used to introduce a defining or identifying relative clause.

Example: animals which hibernate
Example: the evidence on which it is based
3. Used to introduce a commenting clause, used chiefly in reference to things or ideas rather than people.

Example: The house, which lies back from the road, is painted red
4. Used in a relative clause, meaning ‘any that'.

Example: Take which books you want
Example: Tell me which you fancy

Idiom: which is which?

Said of two or more virtually indistinguishable people or things, etc: which is the one, which is the other?

Etymology: Anglo-Saxon hwilc.