In-degree

The in-degree of a vertex $v$ in a directed graph is the number of edges directed into it, counting multiplicities when parallel edges are allowed. A directed loop contributes one to both the in-degree and the outdegree.

For edges $a\to b$, $c\to b$, and $b\to c$, the vertex $b$ has in-degree two and outdegree one. In every finite directed graph, the sum of the in-degrees equals the number of edges, since every edge has exactly one destination. It also equals the sum of the outdegrees.