Boyle's law is applicable for an   [NCERT Exemplar]

Boyle's law is applicable when temperature is constant.
i. e.                              pV = nRT = constant

$\Rightarrow$             pV = constant         (At constant temperature)

i.e.               $p\propto \frac{1}{V}$     (where, p = pressure and V = volume)

So, this process can be called as isothermal process.