noun
tangent plane; tangential plane
Technical term in mathematics, especially in differential geometry, for the plane that touches a surface at a point.
曲面の接平面を求める。
Find the tangent plane of the curved surface.
接線 is a tangent line (1D), while 接平面 is a tangent plane (2D).
Compound of 接 (contact, tangent) + 平面 (plane). A straightforward technical term.