DAG Directed acyclic graph
A graph with directed edges and no cycles — the data structure behind build systems, version control and every pipeline tool in existence.
The absence of cycles is what makes it useful: a DAG can always be topologically sorted, i.e. flattened into a valid execution order. That is why dependency graphs, task schedules and build plans are DAGs.
Concretely: Git's commit history is a DAG (hence merge commits with two parents), Airflow and Dagster call their pipelines DAGs outright, and Spark's execution plan is a DAG of transformations. Detecting a cycle is by definition an error.