A best approximation to a real number $x$ for the denominator bound $d$ is a rational number $\frac{r}{s}$ (in reduced form) with $s ≤ d$, so that any rational number $\frac{p}{q}$ which is closer to $x$ than $\frac{r}{s}$ has $q > d$.
Usually the best approximation to a real number is uniquely determined for all denominator bounds. However, there are some exceptions, e.g. $\frac{9}{40}$ has the two best approximations $\frac{1}{4}$ and $\frac{1}{5}$ for the denominator bound $6$. We shall call a real number $x$ ambiguous, if there is at least one denominator bound for which $x$ possesses two best approximations. Clearly, an ambiguous number is necessarily rational.
