Definition:Planar Graph/Face

Definition

 * PlanarGraph.png

The faces of a planar graph are the areas which are surrounded by edges.

In the above, the faces are $ABHC$, $CEGH$, $ACD$, $CDFE$ and $ADFEGHIHB$.

Adjacent
In the above diagram, $ABHC$ and $ACD$ are adjacent, but $ABHC$ and $CDFE$ are not adjacent.