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A ground atomic formula or GAF (rhymes with “staff”) is a Cyc formula of the form (predicate arg1 [arg2 ...argn]), where the arguments are all terms of any kind, but not variables. For example:

(likesAsFriend ThomasJefferson JohnAdams)

(eats GeorgeWashington (FruitFn AppleTree))

(beliefs GeorgeWashington
  (likesAsFriend ThomasJefferson JohnAdams))

GAFs are a subset of atomic formulas. They are those atomic formulas in which no variables appear.