Define stopping potential (cut-off potential) with reference to the photoelectric effect.

Hint: Photoelectric effect is the ejection of an electron when electromagnetic radiation or photon is absorbed by the electron. For the electron to be ejected the absorbed must have a frequency higher than the threshold frequency of the material. Threshold Energy is the minimum energy required by an electron for ejection from the metal surface, it is also called the photoelectric work function.

Complete step-by-step answer:
When light or a photon with optimum energy hits a metal surface, the surface of the metal ejects electrons by absorbing the photon of optimum energy. This effect is called the Photoelectric Effect.
For the electron to be ejected the photon must have energy equal to or more than the threshold energy of metal. The ejected electron is called a Photoelectron. The extra energy of the photon, that is the energy it has on top of the threshold energy, gets transferred to the photoelectron in the form of Kinetic Energy.
In the Photoelectric Experiment, light or photon is directed towards a metal plate, called the emitter, and if the photons have energy equal to or more than the threshold energy of metal, the photoelectron is ejected. The ejected electron, with Kinetic Energy from the photon, then moves to a second metal plate, called the collector, that is kept across the emitter plate. This movement of photoelectrons from emitter to the collector causes a current called the Photoelectric current.
As we already know that electrons move from lower potential to higher potential. So, the collector is connected to the negative end of a battery to disrupt the flow of photoelectrons, or to stop the Photoelectric Current, due to the voltage or Potential Difference across the plates. The Stopping Potential or the Cut-off Potential is the potential difference at which the ejected photoelectrons stop moving across the plates or in other words, The Photoelectric Current between the plates falls to 0.

Note: As Stopping Potential counters, the movement of photoelectrons possessing Kinetic Energy. It is also used as a tool to measure the Kinetic Energy of an electron. The product of the charge on an electron and the stopping voltage gives us the maximum kinetic energy of that ejected electron.