An isolated system is one where, for all intents and purpose, you can disregard interaction with any object outside the system as irrelevant.
In thermodynamics, it usually means a box that allows no heat transfer between the Exterior and interior side.
Exterior forces include the applied force, normal force, tension force, friction force, and air resistance force.
So in short we can say when applied force is acting on a body which doesn't allow any transfer of heat between d internal and external contents of a body.