tree
A {directed acyclic graph}; i.e. a {graph}
wherein there is only one route between any pair of {nodes},
and there is a notion of "toward top of the tree" (i.e. the
{root node}), and its opposite direction, toward the {leaves}.
A tree with n nodes has n-1 edges.
Although maybe not part of the widest definition of a tree, a
common constraint is that no node can have more than one
parent. Moreover, for some applications, it is necessary to
consider a node's {daughter} nodes to be an ordered {list},
instead of merely a {set}.
As a data structure in computer programs, trees are used in
everything from {B-trees} in {databases} and {file systems}, to
{game trees} in {game theory}, to {syntax trees} in a human or
computer {languages}.
(1998-11-12)