In Prolog, defining a simple rule involves stating a relationship between one or more variables. This is done by using the :- operator, followed by the predicate that must be true in order for the rule to be true. For example, if we want to define a rule that states "X is the parent of Y if X is the mother of Y or X is the father of Y", we can write:parent(X,Y) :- mother(X,Y); father(X,Y).This rule states that X is the parent of Y if X is the mother of Y or X is the father of Y.