tangent plane

tangent plane
    n 1: the plane that contains all the lines tangent to a specific
         point on a surface
    

Tangent \Tan"gent\, a. [L. tangens, -entis, p. pr.]
   Touching; touching at a single point; specifically (Geom.)
   meeting a curve or surface at a point and having at that
   point the same direction as the curve or surface; -- said of
   a straight line, curve, or surface; as, a line tangent to a
   curve; a curve tangent to a surface; tangent surfaces.
   [1913 Webster]

   {Tangent plane} (Geom.), a plane which touches a surface in a
      point or line.

   {Tangent scale} (Gun.), a kind of breech sight for a cannon.
      

   {Tangent screw} (Mach.), an endless screw; a worm.
      [1913 Webster]
      [1913 Webster]