(mathematics) A number expressed in the decimal system (base ten), (particularly) fractional numbers so represented.

(algebra, number theory) A complex number (more generally, an element of a number field) that is a root of a polynomial whose coefficients are integers; equivalently, a complex number (or element of a number field) that is a root of a monic polynomial whose coefficients are rational numbers.

(mathematics) Any real number that cannot be expressed as a ratio of two integers.

(physics) The ratio of the velocity of a body to that of sound in the surrounding medium.

(chemistry) The hypothetical charge of an atom within a molecule.

(computing) The character # used to represent the word number; the hash sign; also called the pound sign in the US

(idiomatic) To figure; to do the math.

(arithmetic) A set of numerals for representing numbers.

A measure of the antiknock properties of an automobile fuel; isooctane having an octane number of 100, a fuel with the same antiknock properties as a mixture of 90% isooctane and 10% heptane having an octane number of 90.

A common alternative form of PIN (in the sense of "personal identification number").