A *network* or *graph* consists of points, and lines
connecting pairs of points. The points are called *nodes* or
vertices. The lines are called *arcs*. The arcs may have a
direction on them, in which case they are called *directed
arcs*. If an arc has no direction, it is often called an
*edge*. If all the arcs in a network are directed, the network
is a *directed network*. If all the arcs are undirected, the
network is an *undirected network*.

Two nodes may be connected by a series of arcs. A *path* is a
sequence of distinct arcs (no nodes repeated) connecting the nodes. A
*directed path* from node *i* to node *j* is a sequence of arcs,
each of whose direction (if any) is towards *j*. An *undirected
path* may have directed arcs pointed in either direction.

A path that begins and ends at the same node is a *cycle* and may
be either directed or undirected.

A network is *connected* if there exists an undirected path
between any pair of nodes. A connected network without any cycle is
called a *tree*, mainly because it looks like one.

Mon Aug 24 16:30:59 EDT 1998