A function (in the mathematical sense) is a relation such that for each thing in its domain (the universe of things it can be applied to), there is at most a single thing in its range (the universe of results it can have) such that the relation holds between the two.
In CycL, functions are denoted by first order reified terms, aka FORTs (constants or NARTs). These terms are referred to as “function-denoting terms”, “CycL functions,” or sometimes just “functions.” CycL functions can be applied to arguments to form non-atomic terms, which can serve as arguments to a predicate just as other terms can.