The waist width is the narrowest point of the ski, measured under your boot in millimeters. It's the single most important measurement for determining where a ski performs best—specifically, how well it grips on firm snow versus how easily it floats in soft snow.
For mostly groomed runs and hardpack, stay under 85mm—these skis are quicker edge-to-edge and hold better on firm snow. The 85–95mm range is a sweet spot for all-mountain skiing, handling both groomers and off-piste reasonably well. If you regularly chase soft snow, 95–105mm gives you float while still managing groomed runs. Anything over 105mm is built for deep powder.
The biggest mistake shoppers make is going wider than they need. Wide skis require more effort to roll onto edge and feel sluggish on hardpack, so if you primarily ski firm conditions, a narrower ski will actually be more fun and easier to control.