What Are Interrogative Pronouns in English?

Definition:

A term in traditional grammar for a pronoun that introduces a question. (Related terms include interrogative, wh-word, and question word, although these terms are generally not defined in precisely the same way.)

In English, who, whom, whose, which, and what commonly function as interrogative pronouns. (When immediately followed by a noun, whose, which, and what function as determiners.)

See also:

Examples and Observations:

Who are you?
"Even if you do learn to speak correct English, whom are you going to speak it to?"
(attributed to Clarence Darrow)
"Yes! and that third place, on the other side, whose is that? – whose, whose, whose?"
(Wilkie Collins, "Mr. Wray's Cash Box," 1852)
"When a man tells you that he got rich through hard work, ask him: 'Whose?'"
(Don Marquis)
“I have water and Diet Coke. That was the only soft drink I allowed Howie to have. Which do you prefer?"
(Stephen King, Under the Dome. Scribner, 2009)
"I got a belt on that's holding up my pants, and the pants have belt loops that hold up the belt. What is going on here? Who is the real hero?"
(Mitch Hedberg)
"Notice that an interrogative pronoun will never have an antecedent. Since it asks a question, there is no way to know which noun an interrogative pronoun refers to. The antecedent remains a mystery until the question is answered."
(M. Strumpf and A. Douglas, The Grammar Bible. Owl Books, 2004)

What versus Which
What is used when specific information is requested from a general or open-ended possible range. Which is used when specific information is requested from a restricted range of possibilities:
A. I've got your address. What's your phone number?
B. Oh it's 267358.
(an open-ended range of possible information)
[looking at a pile of coats]
A. Which is your coat?
B. That black one.
However, where the number of options is shared knowledge among speakers and listeners, what + noun is often used in informal contexts. Here, what is an interrogative pronoun used as a determiner:
[talking about a shop]
What side of the street is it on, left or right?
(or: Which side of the street is it on?)

A: Did you see that documentary about the SARS virus last night?
B; No, what channel was it on?
(or: Which channel was it on?)
(R. Carter and M. McCarthy, Cambridge Grammar of English: A Comprehensive Guide. Cambridge Univ. Press, 2006)