APOs

APOs is an acronym used in several domains, with the two most common references being Army Post Office and apochromatic optics. Because APO can denote different concepts, the intended meaning depends on context.

Army Post Office: An APO designates a mail processing and delivery location used by the U.S. armed forces and some allied services to send mail to personnel stationed overseas. APO addresses route mail through the military postal system rather than via civilian post offices. An APO address typically includes a region code such as AE (Europe, the Middle East, and Africa), AP (the Pacific), or AA (the Americas), followed by a postal code and the recipient’s name and unit. Mail can be sent with standard postage, though some restrictions may apply for hazardous items or perishable goods. The system supports letters, parcels, and care packages and is important for morale and logistics in deployed operations.

Apochromatic optics: APO stands for apochromatic, describing lenses designed to correct chromatic aberration for multiple wavelengths, typically reducing color fringing across red, green, and blue light. Apochromatic designs aim to bring these wavelengths to a common focal plane, improving sharpness and color fidelity. They are common in high-end photography lenses and astronomical telescope objectives, often featuring triplet or quadruplet configurations and special glasses such as low-dispersion (ED) glass. The APO designation signals superior color correction compared with standard achromats and is used by manufacturers as a quality indicator.

In other contexts, APO can have additional, less common meanings. Context usually determines the intended interpretation.