noun which may take the genitive case particle 'no'
curved; arched; mountain-shaped
Describes something that rises and falls in a smooth curve like a mountain, often used for trajectories, eyebrows, or architectural shapes.
ボールは山なりの軌道を描いて飛んでいった。
The ball flew in a high, arched trajectory.
彼女は山なりの眉をしている。
She has arched eyebrows.
アーチ状 specifically means 'arch-shaped' and is more technical or architectural, while 山なり is a more everyday, visual comparison to a mountain's curve.
Compound of 山 (mountain) and なり (shape/form), literally 'mountain-shaped'. The reading やまなり is straightforward.