Terminal voltage is the voltage output measured across its terminals of a voltage source.(a single battery can be considered as a voltage source).
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.