SHIELD was the elite anti-espionage agency of the Marvel universe. Originally it was mainly concerned with the activities of Hydra but over time stuck their noses into just about everybody’s business. The name stood for Supreme Headquarters, International Espionage and Law-Enforcement Division. At least at first it did. Later it became Strategic Hazard Intervention Espionage Logistics Directorate. With its introduction into the cinematic universe they settled on Strategic Homeland Intervention, Enforcement and Logistics Division. Regardless, the entire organization has been retired in favor of a better acronym.
References: [wikipedia.org: S.H.I.E.L.D.]
[marvel.fandom.com: Strategic Homeland Intervention Enforcement and Logistics Division]
[Dr. Strange in the Multiverse of Madness] [2022] [ref to ; ]
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Metahuman Database 