A beard-second is a measurement of length based on the distance that a beard hair grows in one second of time. This is similar, in structure though not in quantitative value, to a light-year, which is a measurement of length equal to the distance that light travels in one year. While a light-year is a very long distance, due to the tremendous speed of light, the beard-second is quite small since hair grows at a very slow rate. The exact length that this equals is the basis for some debate, and some sources cite it at about 10 nanometers (nm) while other sources use only 5 nm.

There are a number of ridiculous or unnecessarily complicated measurements used, often unofficially, in physics and engineering. In quantum physics, and other disciplines that use extremely small measurements of distance and space, one such measurement is the beard-second. There are two different values that can be used for the beard-second, based on the fact that beard follicles can grow at different rates on different people. The larger of the two measurements is 10 nm, while the smaller measurement is 5 nm in length; a nanometer is one-billionth of a meter (m), or about three-billionths of a foot, in length.