For an undirected graph, we know that nodes are adjacent to each other if there is a link that connects them. What about adjacency for directed graphs? Is it based on:

- outgoing links: node $n$ is adjacent to node $i$ if there is a link coming out from node $n$ to node $i$
- ingoing links: node $n$ is adjacent to node $i$ if there is a link coming out from node $i$ to node $n$
- either: mixed of both above options (like adjacency in undirected graphs)

Any references would be appreciated.