Polygraph (mathematics)
In mathematics, and particularly in category theory, a polygraph is a generalisation of a directed graph. It is also known as a computad. They were introduced as "polygraphs" by Albert Burroni and as "computads" by Ross Street. In the same way that a directed multigraph can freely generate a category, an n-computad is the "most general" structure which can generate a free n-category. References 

Category theory
Directed graphs