The Collaborative International Dictionary of English v.0.48:

Interrogative \In`ter*rog"a*tive\, a. [L. interrogativus: cf. F.
   interrogatif.]
   Denoting a question; expressed in the form of a question; as,
   an interrogative sentence; an interrogative pronoun.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Interrogative \In`ter*rog"a*tive\, n. (Gram.)
   A word used in asking questions; as, who? which? why?
   [1913 Webster]

WordNet (r) 3.0 (2006):

interrogative
    adj 1: relating to verbs in the so-called interrogative mood;
           "not all questions have an interrogative construction"
    2: relating to the use of or having the nature of an
       interrogation [syn: interrogative, interrogatory] [ant:
       asserting(a), declarative, declaratory]
    n 1: a sentence of inquiry that asks for a reply; "he asked a
         direct question"; "he had trouble phrasing his
         interrogations" [syn: question, interrogation,
         interrogative, interrogative sentence]
    2: some linguists consider interrogative sentences to constitute
       a mood [syn: interrogative mood, interrogative]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "interrogative":
   bone of contention, catechism, cross-interrogatory, cross-question,
   debating point, demand, feeler, inquiry, interrogation,
   interrogatory, issue, leader, leading question, moot point,
   point at issue, point in question, problem, query, question,
   question at issue, question mark, quodlibet, topic, trial balloon,
   vexed question