(fluid mechanics, meteorology) A dimensionless number expressing the ratio of buoyancy to flow shear in a fluid flow.

the number of particles per unit volume

(mathematics) A positive even integer that can be expressed as the sum of two odd primes; involved in Goldbach's conjecture.

(mathematics) A positive integer which is divisible by the sum of all of its digits.

(mathematics) Any integer which is the product of some integer with the integer itself.

(mathematics) A positive integer that is divisible by every one of its digits.

a phone number (on a phone call) that is unfamiliar to the recipient / a hidden number; a phone number that is hidden from the recipient

(geometry) The maximum number of layers of copies of the same shape that can surround a given shape.

A numerical address for postal correspondence, managed by a company or organisation on behalf of the addressee, and used e.g. for convenience, if one does not have a suitable physical address for receiving mail, or to avoid giving one's real address (e.g. in a newspaper advertisement).

(optics) A dimensionless quantity (denoted F), such that for an electromagnetic wave passing through an aperture and hitting a screen, F=(a²)/(L𝜆) where a is the characteristic size (e.g. radius) of the aperture, L is the distance of the screen from the aperture, and 𝜆 is the incident wavelength.