Mode field diameter
In telecommunication, a mode field diameter (MFD) is an expression of distribution of the irradiance, i.e., the optical power, across the end face of a single-mode fiber.
Note: For a Gaussian power distribution in a single-mode optical fiber, the mode field diameter is that at which the electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power is reduced to 1/e2 of the maximum power, because the power is proportional to the square of the field strength.
Source: from Federal Standard 1037C