Fresnel number

Etymology
Named after the physicist.

Noun

 * 1)  A dimensionless quantity (denoted $$F$$), such that for an electromagnetic wave passing through an aperture and hitting a screen, $$F = \frac{a^{2}}{L \lambda}$$ where  $$a$$ is the characteristic size (e.g. radius) of the aperture, $$L$$ is the distance of the screen from the aperture, and $$\lambda$$ is the incident wavelength.