1 of a node in a tree. The length of the unique path from the root of the tree to the node. Thus if a node A is the root node then its depth is zero, otherwise its depth is one greater than that of its parent.
In some texts, depth of a node is synonymous with level of a node.
2 of a tree. The maximum depth of any node in a tree. The depth of a given tree will have the same numerical value as the height of that tree.