Mean degree of a node's neighbors (Maslov & Sneppen 2002), the
"average neighbor degree" reported by Cytoscape:
$$C_{NC}(i) = \frac{1}{k_i} \sum_{j \in N(i)} k_j.$$
High values mark nodes attached to hubs. Isolates score 0. Under
mode = "out" the out-neighbors' out-degrees are averaged, under
"in" the in-neighbors' in-degrees.