perimeter

continuous line forming the boundary of a shape not including shared pixels, or the minimum bounding box (opens in a new tab), whichever is shortest.

Example: The perimeter calculation for a 2 CSS pixel perimeter around a rectangle is 4h+4w, where h is the height and w is the width. For a 2 CSS pixel perimeter around a circle it is 4𝜋r.

Source: WCAG 2.2 definition of “perimeter” (opens in a new tab) (W3C)