One more than twelve; ten and three; as, thirteen ounces or pounds. [1913 Webster]

1.  The number greater by one than twelve; the sum of ten and three; thirteen units or objects. [1913 Webster]

2.  A symbol representing thirteen units, as 13 or xiii. [1913 Webster]

thirteen, n. & adj.
--n.
1 one more than twelve, or three more than ten.
2 a symbol for this (13, xiii, XIII).
3 a size etc. denoted by thirteen.
--adj. that amount to thirteen.

Derivative:
thirteenth adj. & n.

Etymology:
OE threotiene (as THREE, -TEEN)

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.