The component that decides who does what and in what order. It might be a model making the decision, or it might be ordinary code following a defined flow. The second option is more boring, more reliable and usually the right answer.
Technical view
Responsibilities include task routing, dependency management, parallelism, retry policy, budget enforcement and result aggregation. A deterministic orchestrator gives testability and predictable cost; a model-driven one gives flexibility for open-ended work. The pragmatic pattern is a deterministic outer loop with model-driven decisions confined to well-defined choice points, which preserves observability while allowing adaptation.
Related
Part of AI Terminology Index, section: Agents and Tools.