The Collaborative International Dictionary of English v.0.48:

Ordnance \Ord"nance\, n. [From OE. ordenance, referring orig. to
   the bore or size of the cannon. See Ordinance.]
   Heavy weapons of warfare; cannon, or great guns, mortars, and
   howitzers; artillery; sometimes, a general term for all
   weapons, ammunitiion, and appliances used in war.
   [1913 Webster]

         All the battlements their ordnance fire. --Shak.
   [1913 Webster]

         Then you may hear afar off the awful roar of his [Rufus
         Choate's] rifled ordnance.               --E. Everett.
   [1913 Webster]

   Ordnance survey, the official survey of Great Britain and
      Ireland, conducted by the ordnance department.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Ordnance Survey
    n 1: the official cartography agency of the British government