noun
Refers to a sharply inclined surface, often in geographical or outdoor contexts.
その山の急斜面を登るのは危険だ。
Climbing the steep slope of that mountain is dangerous.
斜面 is a general slope, while 急斜面 specifically means a steep slope.
Compound of 急 (きゅう, 'steep') and 斜面 (しゃめん, 'slope').