kerf

The kerf is the width of the gap a saw leaves behind — the slot its teeth have removed. It is wider than the blade plate itself because the teeth are set: each tooth is bent slightly left or right, so the blade cuts a track wider than the metal it is made of. Without that set the plate would rub in its own cut, bind, and burn the wood or kink the blade.

The kerf is why a cut costs you material. Saw a 1-1/2 inch board in half with a blade whose kerf is 1/8 inch and the halves are not 3/4 inch each: the blade took the difference. Cutting four shelves from one board has to allow for four kerfs, not four knife lines, which is why a plan that ignores the kerf comes up short at the last piece.

On a circular saw the blade projects about one tooth below the work — deep enough to cut through, shallow enough that a sawed face is not splintered. On a table saw the splitter or riving knife sits in the kerf to keep the cut from closing on the blade.