The linearity discussed in this context refers to the relationships of physical quantities from electronics. A linearity of physical quantities like voltages or currents is given, if an output quantity behaves completely proportional to the input quantity, every input voltage change results in a proportional output current or output voltage change.
In the context, it is important that the changes do not depend on the absolute level, but that the same change occurs at a low level as at a high level. For example, if an input voltage change of 1 V results in an output current change of 100 mA, then an input voltage change of 2.5 V must result in an output current change of 250 mA. In this case the change is linear, but for any other output current change it would be nonlinear. Graphically, linearity is shown as a deviation from an ideal straight line.
Most sensors generate a voltage from the physical measurement signal that is nonlinear to the measurement signal. Often, sensor characteristics are empirical curves that apply to a specific range of magnitudes. From these nonlinear physical functions, a proportional output function must be formed by linearization for further processing of the measurement signal. Such linearizations can be reproduced by inverse characteristic curves in analog circuit techniques or by A/D converters with subsequent digital processing.