Acclivous, meaning upward sloping, can be paired with several antonyms based on the direction of the slope. The antonym for an upward sloping terrain could be a downward sloping surface, which is usually referred to as declivous. In contrast, a horizontal slope, without any ascending or descending gradient, could be described as level, flat or planar. If the slope is gentle or very slight, the antonym could be steep or precipitous, meaning sharply sloping. Other suitable opposite words for acclivous could include low, downgrade or descending, which relate to a surface that is moving in a downward direction.