河阶，亦称阶地、河阶地或堆积坡，即是河流下切作用造成河谷斜坡形成台地地形，阶地从一阶到多阶的河阶地形皆有，则视阶地地质与河流侵蚀能力而定。每一阶的阶地均包含阶地面、阶地前面斜坡、阶地前缘、阶地后缘和阶地后方陡坡等部份。阶地高度常以阶地面与河流丰水期水面之间的垂直距离做为高度依据。

阶地面是阶地的表面，实际是原来的河床，受河流下切侵蚀而露出形成台地，大多均向河谷方向倾斜。

阶地前面斜坡是阶地面以下的坡地，阶地面和阶地前方斜坡是组成阶地的两个主要部分，但其发育过程却不相同。

Fluvial terraces are elongated terraces that flank the sides of floodplains and fluvial valleys all over the world. They consist of a relatively level strip of land, called a “tread,” separated from either an adjacent floodplain, other fluvial terraces, or uplands by distinctly steeper strips of land called “risers.” These terraces lie parallel to and above the river channel and its floodplain. Because of the manner in which they form, fluvial terraces are underlain by fluvial sediments of highly variable thickness.