What does SPECTRE stand for?

Special Executive for Counterintelligence, Terrorism, Revenge, and Extortion - Fictitious organization whose agents were characters in the late Ian Fleming's "James Bond" mysteries

Special Executive for Counterintenigence, Terrorism, Revenge and Extortion (SAUS)

Special Radiation Experiment - Marine science

Spectral Radiance (or Radiation) Experiment (SAUS)

Spectral Radiation Experiment (USDC)