In Airflow, pipelines are represented as DAGs (Directed Acyclic Graphs) defined in Python. A graph is a structure composed of objects (nodes) in which certain pairs of objects are related. They are "Directed", which means that the edges of the graph are oriented and that they, therefore, represent unidirectional links. "Acyclic", because the graphs do not have a circuit. This means that node B downstream of node A cannot also be upstream of node A. This ensures that pipelines do not have infinite loops.