What is the residual voltage?

The residual voltage is the voltage that remains in an output transistor while it is ON. For example, consider an NPN output with a power supply voltage of 24 V and a residual voltage of 2 V. While the output transistor is ON, there will be 2 V between the output line (the black wire) and the 0-V line (the blue wire).

Subsequently, one may also ask, what do you mean by residual voltage of LVDT?

Differencing the two secondary voltages results in “zero” output volts. This is the null position of the LVDT. In reality, there is a small residual voltage left due to factors like winding capacitance and variances in the magnetic materials.