An agent with its own identity and permissions that continues working between user interactions, spanning the applications and systems a person uses. The operating change is that work can start without a prompt, which is a different model from chat and requires answers to questions chat never posed — what may it initiate, and how does anyone stop it.
Scheduled automation has always run without a prompt, and it ran a fixed script. What is different is an agent deciding what to do next based on context, which removes the predictability that made scheduled jobs easy to reason about. The gain in flexibility is paid for in the loss of a fixed script somebody could read.