When a system undergoes a physical change under the condition that the temperature of the

system remains constant, then such a process is called isothermal.
For such a process to take place it is necessary that the system be surrounded by a perfectly conducting material.
In practise there is no material which is perfectly conducting, hence perfect isothermal process is impossible.
But when a process takes place very slowly, so that heat finds sufficient time to flow, then the process is approximately isothermal. While adiabatic process is a rapid process.