Snow bridge is an arc across a crevasse

Crevasse

A crevasse is a deep crack in an ice sheet rhys glacier . Crevasses form as a result of the movement and resulting stress associated with the sheer stress generated when two semi-rigid pieces above a plastic substrate have different rates of movement...

, a crack in rock, a creek

Stream

A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...

, or some other opening in terrain. It is typically formed by snow drift, which first creates a cornice

Cornice (climbing)

A snow cornice or simply cornice is an overhanging edge of snow on a ridge or the crest of a mountain. They form by wind blowing snow over the crest of the mountain, so they often form on the leeward sides of mountains...

, which may gradually grow to reach the other side of the opening.

Dangers

A snow bridge may completely cover the opening and thus presents danger: it creates an illusion of unbroken surface while hiding the opening under a layer of snow of unknown thickness, possibly only several centimetres thick.

Snow bridges may also be formed inside a crevasse making it appear shallow.

Since a snow bridge is thicker and stronger at the edge of a crevasse, a fall through a bridge usually happens at some distance from the crevasse edge.