What is transformer ratio ?

The ratio of number of turns in secondary to primary of transformer is defined as transformer ratio
Transformer ratio = $\frac{\text{ output emf }}{\text{ input emf }}=\frac{\text{ No. of turns in secondary }}{\text{ No. of turns in primary }}$
$\text{ i.e }\mathrm{r}=\frac{{\mathrm{N}}_{\mathrm{S}}}{{\mathrm{N}}_{\mathrm{p}}}=\frac{{\mathrm{V}}_{\mathrm{S}}}{{\mathrm{V}}_{\mathrm{p}}}$