The depth of a box is the horizontal distance from front to back (usually not necessarily defined to be smaller than the width, the horizontal distance from side to side).
Depth
See also
Depth-First Traversal, Distance, Graph Thickness, Height, Length, Tree Depth, WidthExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Depth." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Depth.html