The Collaborative International Dictionary of English v.0.48:

Interrogation \In*ter`ro*ga"tion\, n. [L. interrogatio: cf. F.
   interrogation.]
   [1913 Webster]
   1. The act or process of interrogating or questioning;
      examination by questions; an instance of interrogating;
      inquiry.
      [1913 Webster]

   2. A question put; an inquiry.
      [1913 Webster]

   3. A point, mark, or sign, thus [?], indicating that the
      sentence with which it is connected is a question. It is
      used to express doubt, or to mark a query. Usually called
      a question mark; called also interrogation point.
      [1913 Webster +PJC]

   Note: In works printed in the Spanish language this mark is
         not only placed at the end of an interrogative
         sentence, but is also placed, inverted [as thus
         ([iques])], at the beginning.
         [1913 Webster]

WordNet (r) 3.0 (2006):

interrogation
    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: a transmission that will trigger an answering transmission
       from a transponder
    3: formal systematic questioning [syn: interrogation,
       examination, interrogatory]
    4: an instance of questioning; "there was a question about my
       training"; "we made inquiries of all those who were present"
       [syn: question, inquiry, enquiry, query,
       interrogation] [ant: answer]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "interrogation":
   Socratic method, asking, bone of contention,
   bringing into question, catechetical method, catechism,
   catechization, catechizing, challenge, cross-examination,
   cross-interrogatory, cross-question, debating point, demand,
   dispute, examination, feeler, grilling, inquiring, inquiry,
   inquisition, interpellation, interrogative, interrogatory,
   investigation, issue, leader, leading question, moot point,
   point at issue, point in question, probing, problem, pumping,
   query, querying, question, question at issue, question mark,
   questioning, quiz, quizzing, quodlibet, seeking, third degree,
   topic, trial balloon, vexed question