Zero proper acceleration is detectable only via accelerometers calibrated using external standards; no purely intrinsic local test distinguishes inertial from non-inertial frames alone.

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Accelerometers(as used in physics): Instruments that detect and measure acceleration—basically devices that tell you how fast something is speeding up, slowing down, or changing direction.
External standards(as used in philosophy of science): Reference points or measurements from outside a system that you use to check or calibrate something—like using a known distance to calibrate a measuring tool.
Inertial frames(as a concept from physics that describes how motion is measured): Imaginary reference points or perspectives from which you measure motion and position; think of them as different viewpoints from which an observer could describe what's happening in space.
Intrinsic(describing the kind of continuities that ground identity): Something that belongs to or is part of something by its very nature, rather than coming from outside or being relational.

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