AR1,
Just the nickname.
A word about nicknames: Tactical Air Command, now Air Combat Command, officially christened the F-16A as the Fighting Falcon. But that name never found wide use on the flightline. As with many aircraft, the unofficial nickname the pilots pinned on the F-16 did catch on: Viper.
Code One Magazine: F-16 Evolution
Featured Articles - F-16 Fighting Falcon, F16, or Viper? :: F-16.net
Lockheed recently used Viper and Super Viper when promoting overseas sales.
Singapore: Lockheed Unveils F-16 Viper Variant | Defense News | defensenews.com