Sentence:  In some neighborhood of an isolated tangent point in the f-plane , say **f , the <WSD>function</WSD> **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:  (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)