Set theoretic definition of pre-image – math.stackexchange.com

Let $f : X \to Y$ and let $B \subseteq Y$. Then according to Wikipedia, the preimage of $B$ under $f$ is defined like \begin{align*} f^{-1}\bigl[B\bigr] = \bigl{ x \in X ;:, f(x) \in B \bigr} ...