A graph comprises of nodes and edges. The nodes can connect to another node when an edge connects the two. The edge can be directed or directional in that it describes a starting node and an ending node. The graph with nodes and directed edges is called a directed graph.

When a directed graph has a node with no outgoing edges, perhaps it has only other nodes connected to it via directed edges, the node can be said to be a graph sink.

An example is the following graph:

`A`

-> `B`

`A`

-> `C`

`B`

-> `C`

The node `C`

has only ingoing edges with no outgoing edges.