Waist width is the narrowest point of the snowboard, measured in millimeters. It's a critical spec because it determines your boot-to-edge leverage and whether your toes or heels will catch the snow—known as toe drag—when you carve.
Your boot size is the biggest factor in choosing the right waist width. If you wear a US men's size 11 or larger, you'll typically need a wide or mid-wide board (250mm or wider) to prevent your boots from hanging over the edges too much. For smaller boots, a narrower board is ideal because it offers quicker, snappier edge-to-edge transitions.
A good rule of thumb is to make sure your boots overhang the edge by no more than about an inch on each side when centered. Also, keep your binding angles in mind: duck or forward angles can naturally pull your toes and heels in, reducing overhang compared to a flat, zero-degree stance.