Directed acyclic graph
From Pirates@Home
A directed acyclic graph (DAG) is a graph of nodes connected by links where the links have a direction (they are directed links) and where following these links never results in a closed cycle which returns to the starting point (thus acyclic).
One common place DAG's are used is in large-scale distributed computing, where the graph represents the workflow of some kind of computational task, with each node representing a sub-task which must be performed to complete the overall task
