Wiktionary
single-valued function
n. (context mathematics English) Any function in which each point in the domain maps to just one point in the range
Wikipedia
Single-valued function
A single-valued function is an emphatic term for a mathematical function in the usual sense. That is, each element of the function's domain maps to a single, well-defined element of its range. This contrasts with a general binary relation, which can be viewed as being a multi-valued function.