A terrain feature that is rounded inward like the inside of a bowl, i.e. goes from more steep to less steep.
Whether a slope is concave, convex, or planar makes some difference in avalanche danger, usually not a significant difference. Avalanches happen on any steep slope without thick anchors despite the shape of the slope. Slope shape makes more difference on smaller slopes than on larger ones.
On small concave slopes, often there is enough compressive support from the bottom to prevent hard-slabs from releasing but on medium to large slopes, compressive support plays very little role.