figure tree consisting of set of nodes and edges sibling nodes in the tree that are children of the same parent are said to be siblings the nodes etcand usrare siblings in the filesystem tree subtree subtree is set of nodes and edges comprised of parent and all the descendants of that parent leaf node leaf node is node that has no children for examplehuman and chimpanzee are leaf nodes in figure level the level of node is the number of edges on the path from the root node to for examplethe level of the felis node in figure is five by definitionthe level of the root node is zero height the height of tree is equal to the maximum level of any node in the tree the height of the tree in figure is two definitions with the basic vocabulary now definedwe can move on to formal definition of tree in factwe will provide two definitions of tree one definition involves nodes and edges the second definitionwhich will prove to be very usefulis recursive definition definition one tree consists of set of nodes and set of edges that connect pairs of nodes tree has the following propertiesone node of the tree is designated as the root node every node nexcept the root nodeis connected by an edge from exactly one other node pwhere is the parent of unique path traverses from the root to each node if each node in the tree has maximum of two childrenwe say that the tree is binary tree figure illustrates tree that fits definition one the arrowheads on the edges indicate the direction of the connection definition two tree is either empty or consists of root and zero or more subtreeseach of which is also tree the root of each subtree is connected to the root of the parent tree vocabulary and definitions