1.  twelfth(n = noun.linkdef) - position 12 in a countable series of things;
is a kind of rank

2.  twelfth(n = noun.quantity) duodecimal, one-twelfth, twelfth part - one part in twelve equal parts;
is a kind of common fraction, simple fraction

   twelfth(s = adj.all) 12th - coming next after the eleventh and just before the thirteenth in position;

1.  Next in order after the eleventh; coming after eleven others; -- the ordinal of twelve. [1913 Webster]

2.  Constituting, or being one of, twelve equal parts into which anything is divided. [1913 Webster]

1.  The quotient of a unit divided by twelve; one of twelve equal parts of one whole. [1913 Webster]

2.  The next in order after the eleventh. [1913 Webster]

3.  An interval comprising an octave and a fifth. [1913 Webster]

twelfth, n. & adj.
--n.
1 the position in a sequence corresponding to the number 12 in the sequence 1-12.
2 something occupying this position.
3 each of twelve equal parts of a thing.
4 Mus. a an interval or chord spanning an octave and a fifth in the diatonic scale. b a note separated from another by this interval.
--adj. that is the twelfth.

Idiom:
Twelfth Day 6 Jan., the twelfth day after Christmas, the festival of the Epiphany. twelfth man a reserve member of a cricket team. Twelfth Night the evening of 5 Jan., the eve of the Epiphany. twelfth part = sense 3 of n.

Derivative:
twelfthly adv.

Etymology:
OE twelfta (as TWELVE)

N  five, five, cinque, quint, quincux, six, half-a- dozen, half dozen, seven, eight, nine, three times three, dicker, ten, decade, eleven, twelve, dozen, thirteen, long dozen, baker's dozen, quintuplet, twenty, score, twenty-four, four and twenty, two dozen, twenty-five, five and twenty, quarter of a hundred, forty, two score, fifty, half a hundred, sixty, three score, seventy, three score and ten, eighty, four score, ninety, fourscore and ten, sestiad, hundred, centenary, hecatomb, century, hundredweight, cwt, one hundred and forty-four, gross, thousand, chiliad, millennium, thousand years, grand, myriad, ten thousand, ban, man, ten thousand years, banzai, lac, one hundred thousand, plum, million, thousand million, milliard, billion, trillion, five, quinary, quintuple, fifth, senary, sextuple, sixth, seventh, septuple, octuple, eighth, ninefold, ninth, tenfold, decimal, denary, decuple, tenth, eleventh, duodenary, duodenal, twelfth, in one's 'teens, thirteenth, vicesimal, vigesimal, twentieth, twenty-fourth, vicenary, vicennial, centuple, centuplicate, centennial, centenary, centurial, secular, hundredth, thousandth.

N  quinquesection, &c, division by five, quinquesection, decimation, fifth, quinquefid, quinquelateral, quinquepartite, quinqevalent, pentavalent, quinquarticular, octifid, decimal, tenth, tithe, duodecimal, twelfth, sexagesimal, sexagenary, hundredth, centesimal, millesimal.

twelfth cranial nerve, twelfth day, twelfth night, twelfth part