(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.

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

(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.

plural of tone number

A number allocated to a syllable in various transliteration systems such as Hanyu Pinyin indicating the tone in which it is pronounced.

A telephone number dialled by mistake, for example by transposing digits. / A telephone call received from an unfamiliar caller, due to a mistake in the number dialled.

(idiomatic) A numbers game: the use of inappropriate statistics (numbers) to reflect a desired result (usually misleading). / plural of number game: lotteries, numbers games.