A unique number, assigned to a particular unit of some product, to identify it.

(mathematics) A complex number consisting of more than two square roots of negative one, creating multiple simulated dimensions.

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

A number written in the form a × bᶜ, e.g. 3.75 × 10³ (a being the mantissa, b, the base and c, the exponent).

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

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

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.

(idiomatic) Some period of time, such as a term of employment or a lifetime, is coming to an end.

A telephone number not listed in the directory.