Inverse Function:A function that reverses another function such that if $f(a) = b$, then its inverse $f^{-1}(b) = a$.
Horizontal Line Test:A graphical method to determine if a function is one-to-one; if any horizontal line intersects the graph more than once, it is not one-to-one.
Injective Function: \text{A function where each element of the range is mapped by exactly one element of its domain.}