The Collaborative International Dictionary of English v.0.48:

Interrogatory \In`ter*rog"a*to*ry\, n.; pl. Interrogatories.
   [Cf. F. interrogatoire.]
   A formal question or inquiry; esp. (Law), a question or
   series of questions asked in writing, usually as part of a
   lawsuit. --Macaulay.
   [1913 Webster]