Thermodynamics, isolated system versus adiabatic process

In thermodynamics, is any processes being done on isolated system will be adiabatic process?

For example, Steam at 400C, 100 bar in an insulated cylinder is suddenly decompressed to 10 bar, by
unlocking the piston and allowing the gas to expand, with a constant external pressure of 10 bar
applied to the piston? Find T2 and entropy change?

Does that mean this decompression is adiabatic decompression in insulated cylinder?

You have to specify that it is adiabatic
and then set up the situation so it is true. That might
mean if the reaction occurs too quickly for the diffusion of heat
or if you have
it taking place in an insulated reactor where heat
cannot be exchanged.

In the above case you specified that it is insulated
and THAT is what makes it adiabatic.