Dictionary

Which of two.

Introducing a direct interrogative question (often with correlative or) which indicates doubt between alternatives.

Used to introduce an indirect interrogative question that consists of multiple alternative possibilities (usually with correlative or).

Example: He chose the correct answer, but I don't know whether it was by luck or (whether it was) by skill.

Without a correlative, used to introduce a simple indirect question.

Example: Do you know whether he's coming?

Used to introduce a disjunctive adverbial clause which qualifies the main clause of the sentence (with correlative or).

Example: He's coming, whether you like it or not.