Leading lady is an informal term for the actress who plays a secondary lead or supporting role, usually a love interest, to the leading actor in a film or play. It is not usually applied to the leading actress in the performance if that actress is the protagonist.
A leading lady is also an actress of renown. For example, Lynn Fontanne and Helen Hayes were both referred to as the 'leading lady of the theatre' in their time. Similarly, Mary Pickford was called the 'leading lady' of the cinema.
The epithet has been applied to an actress who is often associated with one particular actor, for example, Olivia de Havilland was Errol Flynn's leading lady in several films, Katherine Hepburn had a similar association with Spencer Tracy. A leading lady is also an actress who is typecast in romantic supporting roles.
The term is also used collectively, as in 'Hollywood's leading ladies' to refer to a group of notable, famous or popular actresses.