Both “ordinance” and “ordnance” are tied in to authority, but very differently. What a difference an “i” makes!
Ordinance is an authoritative degree, a municipal regulation (a law created by a governmental authority), or something decreed by fate or a deity. Think “ceremony,” “practice,” “law,” or “prescribed usage.”
Ordnance is a military term. It means military supplies, including weapons, ammunition, combat vehicles, and maintenance tools and equipment. Ordnance is also a service of an army charged with the procuring, distributing, and safekeeping of ordnance. Think “cannon,” “artillery.”