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Informally, a rule or conditional is any CycL assertion whose sentence begins with #$implies. Typically, a rule has two parts: an antecedent and a consequent, or left-hand side and right-hand side>. For example,

       (isa ?BIRD FlightlessBird)   
       (behaviorIncapable ?BIRD Flying-FlappingWings doneBy))

However, there are rule assertions which do not satisfy the above. For example, this rule says that any ?OBJ in our universe of discourse, without condition, is an instance of #$Thing:

(isa ?OBJ Thing)

This is also a rule:

           (isa ?ACT Flying-FlappingWings)   
           (doneBy ?ACT ?THING)   
           (isa ?THING FlightlessBird)))

So, a more accurate definition of “rule” in CycL is, “Any assertion whose sentence has free variables.”

For more information on rules in Cyc, see Writing and Editing Rules.