Bijection
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A bijection is a 1:1 map. A map which is both injective and surjective.
It has the useful property that for [math]f:X\rightarrow Y[/math] that [math]f^{-1}(y)[/math] is always defined, and is at most one element.
Thus [math]f^{-1}[/math] behaves as a normal function (rather than the always-valid but less useful [math]f^{-1}:Y\rightarrow\mathcal{P}(X)[/math] where [math]\mathcal{P}(X)[/math] denotes the power set of [math]X[/math])
