An adjacency list is a two-column matrix that abstractly represents the edge relations of
a graph without needing to physically draw the graph.
Each row of the adjacency list encodes an edge

For example, consider the following graph:

The adjacency list of this graph is given below (the rows could be given in any order, and the elements of any given row could be transposed).

1 2 1 5 2 3 2 5 3 4 4 5 4 6

If a graph is instead directed, then to encode the directed edge

The following adjacency list for this graph correctly encodes the orientation of each edge; the edges may be given in any order, as long as the tail of each edge is provided in column 1.

3 8 3 10 5 11 7 8 7 11 8 9 11 2 11 9 11 10