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.

Sun Jun 14 13:20:08 EDT 1998