noun
adjacent side
In geometry, the side of a right triangle that forms the angle in question, together with the hypotenuse.
直角三角形の隣接辺の長さを求める。
Find the length of the adjacent side of the right triangle.
斜辺 is the hypotenuse, the side opposite the right angle, distinct from the adjacent side.
Compound of 隣接 (rin setsu, 'adjacent') and 辺 (hen, 'side'). A straightforward technical term in geometry.