Definition - What does Root Node mean?
A root node is either the topmost or the bottom node in a tree data structure, depending on how the tree is represented visually.
The root node may be considered the top if the visual representation is top-down or the bottom if it is bottom-up. The analogy is that the tree starts at the roots and then goes up to its crown, so the first node is considered the root.
Techopedia explains Root Node
In a tree data structure, the root node is the very first or parent node. Generally, nodes may have parent and children nodes, but because the root note is the first node, it only has children nodes.
A root node is just like any node, in that it is part of a data structure which consists of one or more fields with links to other nodes and contains a data field; it simply happens to be the first node. In this regard, any node can be a root node in relation to itself and its children if that section of the tree is objectively selected.