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]