What does SHIELD stand for?

Silicon Hybrids with Infra-red Extrinsic Long wavelength (SAUS)

Silicon Hybrids with Infrared Extrinsic Long-Wavelength Detectors

Supreme Headquarters, International Espionage Law Enforcement Division (SAUS)

Supreme Headquarters, International Espionage Law-Enforcement Division - Organization in comic book "Nick Fury, Agent of SHIELD"

Sylvania High Intelligence Electronic Defense (SAUS)

Sylvania High-Intelligence Electronic Defense (MCD)