I have encountered 2 answers for this question. 1. In the natural sciences an isolated system is a physical system without any external exchange – neither matter nor energy can enter or exit, but can only move around inside. Truly isolated systems cannot exist in nature, other than possibly the universe itself, and they are thus hypothetical concepts only. It obeys, in particular, to the first of the conservation laws: its total energy - mass stays constant. and 2. An isolated system is a system that is now acted upon by external forces; hence every force in the system has an equal and opposite action-reaction pair force within the system. So every impulse is balanced by an equal and opposite impulse, leading to conservation of momentum within a system. Whats the best way to define it?