(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").

(chemistry) The number of other atoms surrounding an atom in a molecule / (inorganic chemistry) The number of ligands surrounding a central metal atom in a coordination compound

(cricket) The batsman who bats last; usually the worst batsman in the side.

A back issue of a magazine, etc. / Anything considered old-fashioned.