Sentence:  In some neighborhood of an isolated <WSD>tangent</WSD> point in the f-plane , say **f , the function **f is either double valued or has no values defined , except at the tangent point itself , where it is single valued . 
Meaning of the word marked with <WSD> tags:  a straight line or plane that touches a curve or curved surface at a point but does not intersect it at that point