it is essentially a system comprising an operation (a routine behavior requiring little thought, e.g., typing) that serves to accomplish an action (a behavior that involves planning, e.g., creating a bibliography) in the minimum meaningful context that provides understanding of the function of the individual in interacting with the environment (e.g., preparing a paper for a university course as part of a network of students). The theory emphasizes a hierarchical structure of activity, object-orientedness, internalization and externalization, mediation (by tools, language, and other cultural artifacts or instruments), and continuous development. Also called activity psychology. 2. a theory proposing that old age is a lively, creative experience characterized by maintaining existing social roles, activities, and relationships or replacing any lost ones with new ones. Compare continuity theory