DAG means Directed Acyclic Graph.
Directed
Tasks have a direction: A → B → C.
Graph
A collection of nodes (tasks) and relationships (dependencies).
Acyclic
There must be no cycle.
Valid:
A → B → C
Invalid:
A → B → C → A
Why? If A depends on B, B on C, and C on A, Airflow can never finish resolving the workflow. Airflow validates DAG structure and rejects cyclic dependencies.
Interview tip: DAG = workflow blueprint; DAG Run = one execution of that blueprint for a logical date / data interval.