The complement of a subset B within a set A consists of all elements of A which are not in B.

The complement of a set S, relative to the universal set U, consists of all elements of U that are not in S.

Suppose A is a subset of S. Then the complement of subset A in S consists of all elements of S that are not in A.

The intersection of two sets A and B consists of all elements that are in A as well as in B.

Event A denotes the complement of event A, meaning that Ā consists of all outcomes in which event A does not occur. Event A and Ā must be disjoint, because it is impossible for an event and its complement to occur at the same time.

The word "hilly" is an adjective that describes a characteristic of the landscape, specifically indicating that the terrain consists of many hills or slopes. In this context, "hilly" is acting as an adjectival complement that provides additional information about the noun it modifies.