pronoun
definition
Which of two.
conjunction
definition
Introducing a direct interrogative question (often with correlative or) which indicates doubt between alternatives.
definition
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.
definition
Without a correlative, used to introduce a simple indirect question.
example
Do you know whether he's coming?
definition
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.