Ratiometric is used to describe an output signal which changes in proportion to a change input or supply voltage.
A typical ratiometric device would be a strain gauge output pressure sensor, where the output sensitivity is described as a ratio between output and input supply voltage.
If the sensor is described as having a 3 mV/V ratiometric output sensitivity, this means that the full span output will increase by 3 millivolts for every 1 volt increase in supply voltage.