Bounding box
The smallest rectangle that fully encloses an object — the basis for layout, hit areas and collision tests.
In the browser, getBoundingClientRect() returns an element's rectangle in viewport coordinates, including all its children. SVG has the equivalent getBBox(). The rectangle is axis-aligned, which means a rotated shape gets a box larger than the shape itself.
Why rectangles: testing whether two rectangles overlap costs four comparisons, while exact geometry costs considerably more. Games and graphics engines therefore always do a coarse box test first and compute exactly only for the pairs that passed. The same concept appears in object detection, where the model draws a box around each thing it found.