Ordinance means a regulation, rule, law, or a public injunction or decree: "It is a town ordinance to curb your dog." "The town council recently debated an ordinance concerning garbage disposal." Ordnance, a shortened form of ordinance, has come to mean (1) artillery, (2) military weapons of any kind, and (3) that branch of a military force engaged in securing and storing supplies of varied kinds: "That recruit was placed in the ordnance department, where he was set to work stacking ammunition."
Dictionary of problem words and expressions. Harry Shaw. 1975.