Definition:Anomalous Cancellation

Definition
Let $r = \dfrac a b$ be a fraction where $a$ and $b$ are integers expressed in conventional decimal notation.

Anomalous cancellation is a phenomenon whereby deleting (that is, cancelling) common digits from the numerator $a$ and the denominator $b$ of $r$, the value of $r$ the fraction does not change.

Also see

 * Anomalous Cancellation on 2-Digit Numbers