A steep gully in alpine terrain. In winter, a couloir is usually filled with snow bound by rocks on either side.
Couloirs can help anchor snow to the slope, but create a serious hazard if an avalanche does occur. They also pose numerous challenges for snowpack evaluation and safe ascent and descent. Often the only reasonable route is climbing straight up the couloir, which can expose the entire group to avalanche danger for considerable periods of time. Sometimes couloirs can be approached from the top, but then knowing the snow conditions in the couloir itself becomes just an educated guess.