An agent is a person, project or role which has the ability, capacity and opportunity to sustainably make decisions within a specific context.
Agency is the ability, capacity and opportunity to sustainably make decisions within a specific context.
Agency is often associated only with humans, but can be extended to a wide variety of organisms-- as well as projects and roles-- by specific projects and collectives.
Agents often act through identifiers which directly represent them in written and digital communications systems. In fact, people rarely distinguish self-representative agents from actual persons, especially in live communications. However, some social contexts may enable each person to use multiple identities with potentially different roles. Also (and quite significantly), collectives perform basic functions in many social contexts.
Agency is fundamental to legal definitions of corporation. (This is a fact, not an endorsement.)
Agents can perform roles, and roles are often directly identified with specific agents; however, agents may decide whether or not to perform roles, except in coercive systems.
Each agent's identity should be unique within each official formal context.
Sustainability is an important trait of agency. For example, sustainable systems often require the ongoing assignment of (sustainably acting) agents to roles which provide systemic functions.
Agency is a trait of (some, but not all) complex adaptive systems-- including persons and collectives-- because such systems have the ability to sustain, regulate and repair their decision-making agency within contexts with greatly variable traits.
Agents may sometimes delegate their agency in specific contexts not only to other complexly adaptive agents, but also to bots or to automated processes which they've personally approved.