At a previous job, we had a fair amount of celery tasks and logic around starting them based on user input or on a schedule, retrying on failures and marking progress or cleaning up state in various databases.
Is that a workflow engine?
Open source analogue would be Apache Airflow.
Abstractly, it's some directed acrylic graph (DAG) that is asynchronously computed, sometimes on a schedule.
Unfortunately, most things fall under DAG. But the framework / engine exists to manage the complexity of the ever-extending pipelines declared by the engineers
Event/push-based workflows also fall under this taxonomy.
DAG may too specific. It's really a dependency graph, that likely has a DAG topology.