(genetics) The normal variation in the number of copies (normally two) of a gene, or of sequences of DNA, in the genome of a specific individual

(US) A pencil which produces a specific shade of dark gray. One can usually identify a number 2 pencil by looking for the number "2" conspicuously emblazoned on the pencil. Often required for test-taking and the completion of official forms.

(graph theory) The number of edges in a minimum edge cover of a graph, often denoted as 𝜌=𝜌(G).

(physics) A Reynolds number at which the flow of a fluid changes from laminar to turbulent.

(US) A nine-digit number issued to citizens, permanent residents, and temporary (working) residents under section 205(c)(2) of the Social Security Act, codified as 42 U.S.C. § 405(c)(2). The number is issued to an individual by the Social Security Administration. Its primary purpose is to track individuals for taxation purposes. Abbreviated as SSN. In recent years the SSN has become a de facto national identification number.

(mathematics, number theory) The branch of number theory in which number-theoretic questions are expressed in terms of properties of algebraic number fields or related objects, and studied using techniques from algebra.

(Canada) A nine-digit number issued by the government of Canada to citizens, permanent residents, and some categories of temporary residents, for official identification in the provision of government services and income taxation. Abbreviation: SIN.

(statistics) The statistical tendency toward a fixed ratio in the results when an experiment is repeated a large number of times; law of averages.

(number theory) An element of a completion of the field of rational numbers with respect to a p-adic ultrametric.

plural of random number generator