hc

the product of planck's constant and the speed of light.
useful when calculating the energy of electromagnetic radiation, which is proportional to the frequency by planck's constant. but frequency itself is velocity divided by wavelength, and the velocity would be the speed of light. hence, E = h*(c/frequency) = hc/frequency.

alice: what's the energy of red (650nm) visible light?
bob: oh you just divide hc by the wavelength