Question

what is terminal voltage?

Answer 1

Terminal voltage is the voltage output of a device is measured across its terminals.Terminal voltage is calculated by V = emf - Ir.

Answer 2

Terminal voltage is the voltage output measured across its terminals of a voltage source.(a single battery can be considered as a voltage source).
$\boxed{V = EMF - IR}$
Where V = terminal voltage
          EMF = electromotive force
          I = current flowing in the circuit
          R = internal resistance of the source
Ideally, terminal voltage, V should be equal to EMF(R=0), but since every battery has some resistance of its own, V is not equal to EMF.
The more the internal resistance of a source, the more is the difference between the terminal voltage and the EMF.