Talcot Parsons, from The Social System:

Reduced to the simplest possible terms, then, a social system consists in a plurality of individual actors interacting with each other in a situation which has at least a physical or environmental aspect, actors who are motivated in terms of a tendency to the “optimization of gratification” and whose relation to their situations, including each other, is defined and mediated in terms of a system of culturally structured and shared symbols.

This is practically an inventory of the elements uncovered in design research.

• Actors (which Parsons divides into ego, equivalent to “the user” in UX parlance and alter, which UX treats as elements of social context.)
• Social context (relationships between actors)
• Physical context (environment)
• Needs (a.k.a. “optimization of gratification”)
• Behaviors (involvement with events and artifacts in physical context)
• Interactions (involvement with actors in social context)
• Mental models (the defining/mediating symbol structure)
• Signs (I’m adding this one: affordances that link mental models with real-world events and artifacts)
• Symbols (I’m adding this one, too: indicators of the value-significance of real-world events and artifacts)