na-adjective
Describes terrain that is dangerously steep and rugged. Often used in literary or descriptive contexts.
その山道は険阻で、登るのが難しかった。
The mountain path was steep and rugged, making it difficult to climb.
noun
Refers to a steep, dangerous cliff or slope. More concrete than the adjectival sense.
登山者は険阻に立ち向かった。
The climbers faced the precipice.
Compound of 険 (steep, dangerous) and 阻 (obstruct, block). The exact historical derivation is uncertain, but the combination conveys the idea of terrain that is both steep and difficult to traverse.