Final answer:
Relatively stable periods in a family's life are described by the term 'family life cycle,' encompassing predictable patterns and stages that families go through over time, adapting to various life events and challenges.
Step-by-step explanation:
Relatively stable periods of time in a family's life are referred to as the family life cycle. This concept is rooted in sociological models and represents a set of predictable steps and patterns families experience over time. The family life cycle comprises different stages, such as the establishment of a family, rearing and launching children, and experiencing an "empty nest" period. As the field of sociology has evolved, the recognition of diverse family structures and life events has led to more fluid models, such as the family life course, which views family events as part of a fluid course rather than in strict stages.