(1) A section of a directory hierarchy, which is also called a directory tree. The subtree typically starts at a particular directory and includes all subdirectories and objects below that directory in the directory hierarchy; that is, any subdirectories or objects connected to the directory or to any lower level of its subdirectories.
(2) A tree structure created by arbitrarily denoting a node to be the root node in a tree. A subtree is always part of a whole tree.