Final answer:
George Vaillant describes psychologically mature coping strategies as those that effectively and adaptively manage stress, contributing to an individual's overall psychological health.
Step-by-step explanation:
According to George Vaillant, the defining feature of a psychologically mature coping strategy is the ability to effectively manage stress through adaptive mechanisms that contribute to healthy psychological functioning.
Vaillant's work implies that mature coping involves an understanding of the reality of a situation, the use of humor, altruism, and the capacity for future-mindedness.
These coping strategies help an individual to adapt constructively to challenges and maintain well-being even in the face of life's inevitable stressors.