If there is no mass in the universe but one isolated mass, can we define "inertia" for this mass?!
Inertia is the resistance to a change in motion of an object when an external force is applied. Inertia is really just another word for mass in a physics sense. So yes, if it has mass it also has inertia.
Every force must have an equal and opposite reaction though, and since there are no other masses to react, there is no way to apply an external force.
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