Native ISO
Any ISO value produced by real analogue gain in the sensor's readout circuitry, as opposed to numbers created by brightening the file in software.
Measured in
ISO
In depth
Native values behave predictably: the noise rises, but the tonality and colour separation hold together. Some dual-gain sensors have two native base points, and jumping to the second one can actually reduce noise compared with the ISO just below it. Knowing where those points sit lets a photographer skip straight to the cleaner setting.
Examples in practice
Many dual-gain cameras have native bases at ISO 100 and ISO 640; a file shot at ISO 640 can carry less shadow noise than one shot at ISO 500.