What does udn stand for?

Ulcerated Dermal Necrosis (SAUS)

Dnieproavia - Ukraine - FAA designator (FAAC)

National Park Service, National Register Division, Washington, DC - OCLC symbol (OCLC)

Ulcerative Dermal Necrosis - Medicine

Underwater Doppler Navigation

Uniao Democratica Nacional - National Democratic Union - Brazil

Union Democrata Nacional - National Democratic Union - El Salvador - Political party (PPW)

Union Democratica Nicaraguense - Nicaraguan Democratic Union - Political party (PD)

Updraft Nebulizer - Medicine (EDAA)