(number theory) Any of a sequence of numbers having the property that the sum of the divisors of each, excluding itself, is equal to the next number in the sequence; in the case of the last number of the sequence, the sum of the divisors, excluding that number, is equal to the first number of the sequence.

A dimensionless number used in heat transfer calculations, giving a simple index of the ratio of the heat transfer resistances inside of and at the surface of a body.

(astronomy) The total number of stars in the observable universe, often estimated to be in the sextillions.

(mathematics) Relating to number theory

(grammar) Grammatical number denoting a quantity of exactly three

Any of the arbitrary alphanumeric codes in a system for labelling optical filters, usually for photographic use.

A code assigned to a student upon entering an educational institution, and used to identify the student in official documents.

Any of several forms of fixed-length identification number on stamps, books, etc.

(mathematics) A positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer (including the untouchable number itself).

(number theory) An odd natural number k such that k⨯2ⁿ+1 is composite for all natural numbers n.