—n.
1 one more than eleven; the product of two units and six units.
2 a symbol for this (12, xii, XII).
3 a size etc. denoted by twelve.
4 the time denoted by twelve o'clock (is it twelve yet?).
5 (the Twelve) the twelve apostles.
6 (12) Brit. (of films) classified as suitable for persons of 12 years and over.
—adj. that amount to twelve.
Phrases and idioms:
twelve-note (or -tone) Mus. using the twelve chromatic notes of the octave on an equal basis without dependence on a key system.
Etymology: OE twelf(e) f. Gmc, prob. rel. to TWO
Useful english dictionary. 2012.