Tectonics
The expressive articulation of a structure's logic through the treatment of its surface and form.
In practice
Tectonics is the way a structure's logic is expressed through its visible form and surface: how joints, supports and loads are shown rather than hidden. It is a key quality in architecture and furniture. A table whose exposed wooden joints show how the top rests on the legs has strong tectonics, while one wrapped in smooth panels conceals them. Graphic designers borrow the idea when they let a layout's grid show.
Often confused with Structure
Structure is the internal organization that holds parts together; tectonics is how that structure is expressed and made readable on the surface.
Try this
Examine a chair or table you own and sketch where its joints and supports are visible and where they are hidden.