General AI term
A system that uses a model to choose steps, tools or actions toward a task.
An agent usually combines a model with instructions, state, tools and a loop that observes results and decides what to do next. The model may draft an answer, call a search or write to another system, depending on the permissions around it.
A structured prompt is especially useful when an agent has consequential actions. State the intent, available knowledge, constraints, process and validation, and keep human approval where the workflow requires it.