Most generally, a literal is a CycL sentence of the form (predicate arg1 [arg2 ... argn]), or its negation, where the number of arguments to the predicate can be any positive integer (but usually not more than 5), and the arguments can be any kind of term. For example,
(likesAsFriend Goolsbey Brandy) (eatsWillingly BillM (FruitFn ?X)) (isa ?CAR Automobile) (performedBy ?ACT ?ORG) (not (performedBy ?ACT ?ORG))
Because it includes negated CycL sentences, the class of literals is a superset of the class of atomic formulas.
Usually, “literal” is used to refer to the atomic formulas that make up the internal representation of any assertion’s sentence. Sentences that are asserted into the KB are converted into conjunctive normal form; the sentence of each single assertion is internally represented as a disjunction of literals. Those literals that would be negated in conjunctive normal form are called negative literals; the rest are called positive literals. GAFs are the subset of positive literals in which there are no variables.