What is the term for the minimum frequency of a photon that can cause photoelectric emission?

The minimum frequency of a photon that can cause photoelectric emission is known as the threshold frequency. This term is crucial in the context of the photoelectric effect, which describes how light can eject electrons from a material, typically a metal. Each material has a characteristic threshold frequency; if the frequency of the incident light is below this threshold, no electrons will be emitted regardless of the intensity of the light.

When photons with a frequency equal to or greater than the threshold frequency strike the surface of the material, they possess enough energy to overcome the work function, which is the minimum energy required to liberate an electron from the material's surface. The relationship between the frequency of the incident light and the kinetic energy of the emitted electrons is linear, as described by Einstein's photoelectric equation.

The other terms related to frequency, such as critical frequency, resonance frequency, and emission frequency, do not apply directly to the phenomenon of photoelectric emission. The critical frequency and resonance frequency would pertain to different contexts in physics and wave phenomena, while emission frequency is a more generic term that doesn’t specifically define the minimum frequency required for photoelectric emission. Thus, threshold frequency is the precise term that fits the definition given in the question.