This article is from the James Bond FAQ, by Michael Reed firstname.lastname@example.org with numerous contributions by others.
Fleming had a real affection for this word. He created a Spectreville in
"Diamonds Are Forever", made the decoding machine in "From Russia, With
Love" a Spektor, and finally, beginning with "Thunderball", created the
SPecial Executive for Counterintelligence, Terrorism, Revenge, and
Extortion, SPECTRE. The leader of SPECTRE was Ernst Stavro Blofeld,
Fleming's most famous villainous creation.
SPECTRE is normally spelled as one word. The original British Cape edition
of Fleming's "Thunderball" made it S.P.E.C.T.R.E. (though the dots were
dropped in Fleming's later books), and that spelling continues to crop up in
various books about the Bond series. The spelling S.P.E.C.T.R.E. also
appeared in the trailer for the film "From Russia With Love". It seems odd
since the "P" does not stand for a word.