 Two circles of non-equal radius, both in the same plane, are said to be tangent to each other if they meet at only one point.Equivalently, two circles, with radii of "ri" and centers at ("xi", "yi"), for "i" = 1, 2 are said to be tangent to each other if


The "tangent plane" to a surface at a given point "p" is defined in an analogous way to the tangent line in the case of curves.It is the best approximation of the surface by a plane at "p", and can be obtained as the limiting position of the planes passing through 3 distinct points on the surface close to "p" as these points converge to "p".