(chemistry, physics) The number of atoms present in 12 grams of isotopically pure carbon-12, being 6.02214076 × 10²³. By definition, the number of elementary entities (atoms or molecules) comprising one mole of a given substance.

(usually used attributively) A simple drawing exercise, intended especially for children, consisting of a line drawing that divides the artwork into distinct spaces, the intended color of each indicated by a number and referenced from a key.

alternative form of color by number

A memorable mobile phone number.

(Canada, US) A toll-free telephone number.

(physics) A dimensionless parameter that determines the behavior of viscous flow patterns. In pipe flow a value less than about 2,000 (known as the critical Reynolds number) produces laminar flow, one above about 3,000 produces turbulent flow (intermediate values produce unpredictable behavior).

The number (street address) assigned to a particular house to distinguish it from other buildings on the same street.

(photography) The ratio of the focal length of a lens to its diameter; a measure of the light-gathering ability of the lens.

Alternative form of Erdős number

(mathematics) A number describing the degree of collaboration between a mathematician and Paul Erdős, defined such that a mathematician who has written a paper with Erdős has an Erdős number of 1, a mathematician who has written a paper with somebody having an Erdős number of 1 has an Erdős number of two, and so on.