NIST (the
National Institute of Standards and Technology) defines it as "A
connected, undirected, acyclic graph" (and "rooted"). Our book does
not require the "undirected" part, and many sources assume that a tree
is directed! This does, unfortunately, add to the
confusion.... An ordered tree
is one where the children are numbered (there is a left to right),
which is important for tree traversal.