1. a school of thought, developed primarily by Soviet psychologists, that focuses on activity in general—rather than the distinct concepts of behavior or mental states—as the primary unit of analysis. In this context, an activity is a nonadditive unit that orients an organism in the world; 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; disengagement theory.