A snowstorm with strong winds and low visibility is called a blizzard. The defining danger is not simply heavy snowfall: wind must blow falling or previously fallen snow so forcefully that visibility becomes very poor for a prolonged period.
In the United States, the National Weather Service definition uses sustained winds or frequent gusts of at least 56 km/h (35 mph), visibility of 400 meters (one-quarter mile) or less, and conditions lasting at least three hours. A ground blizzard can happen even without new snowfall when wind lifts and drives loose snow across the surface.
This is why a snowy day is not automatically a blizzard. A snowstorm may produce deep accumulation with relatively calm air, while a blizzard can sometimes have modest new snowfall but extremely dangerous drifting snow. Whiteout describes the visibility condition, not the full storm category; a nor’easter is a large coastal storm that may, but does not always, create blizzard conditions.