The word isothermal refers to the same temperature. So one isothermal transformation of a gas occurs when the initial temperature is conserved.
The physical law that expresses this relationship is known as Boyle's Law and is mathematically expressed by:
p = pressure
V = volume
= constant that depends on the mass, temperature and nature of the gas.
Since this constant is the same for the same gas, when transformed, the relationship is valid:
Certain gas contained in a 1m³ container with piston exerts a pressure of 250Pa. When being isothermally compressed to a volume of 0.6m³ what will be the pressure exerted by the gas?