A set of nodes connected to one another by lines (edges) in which each connection has a specified direction such that no route that follows the direction of the connections enters a loop (cycle).
The SNOMED CT subtype hierarchy is an example of a Directed Acyclic Graph. SNOMED CT concepts are nodes and "is a" Relationships are the directed lines that connect them. All "is a" Relationships lead from a more specific concept to a more general concept , so a cycle would be a logical error (e.g. if "rubella virus" is a type of "virus" and "virus" is a type of "microorganism", then "microorganism" cannot be a type of "rubella virus").