d21

D21 is a designation used in multiple, unrelated domains. In aerospace, D-21 refers to a Lockheed-developed reconnaissance drone from the 1960s. The drone was designed to be launched from a high-speed, high-altitude carrier aircraft and used to perform covert, overland reconnaissance missions. The program was part of Cold War-era efforts to gather imagery without risking manned flights.

In transportation, D21 is used as a road designation in several countries, labeling regional or national routes. The exact route and jurisdiction vary by country, and such numbers can change as road networks are reorganized.

In other contexts, D21 may appear as a model or part number in various consumer electronics, industrial equipment, or corporate product lines. As with many alphanumeric designations, its meaning is highly context-dependent and cannot be determined without domain information.