The term is often used loosely to mean "an agent with a scheduler," but the scheduler is the trivial part. What separates an autonomous system from a script on a timer is everything after the trigger: how it decides what to work on, what process it follows, where it is forced to stop for a human, and what evidence it leaves behind.
A useful test: if the loop ran forty times unattended, could you say afterwards what it did, why, under which rules, and who approved the parts that needed approval? If the answer lives in scattered logs and prompt files, you have automation. If it lives in structure the system itself enforces and records, you have an autonomous system.
In ConvOps, an autonomous system is typically one workspace: its workflows define the process, schedules start the loop, pools select the work, gates hold it where humans are required, and the audit records every mutation.