Idempotence is a semantically overloaded term in computer science where in functional programming it refers to the same concept as mathematical idempotence it refers to any function leading to the same state in multiple calls as the first.

And yes, in real machines we can't ever have true same states between multiple calls as system time, heat and other effects will differ but we define the state over the abstracted system model of whatever we are modelling and we define idempotency as the same state over multiple calls in that system.