The Collaborative International Dictionary of English v.0.48:

Dozen \Doz"en\ (d[u^]z"'n), n.; pl. Dozen (before another
   noun), Dozens (d[u^]z"'nz). [OE. doseine, dosein, OF.
   doseine, F. douzaine, fr. douze twelve, fr. L. duodecim; duo
   two + decem ten. See Two, Ten, and cf. Duodecimal.]
   1. A collection of twelve objects; a tale or set of twelve;
      with or without of before the substantive which follows.
      "Some six or seven dozen of Scots." "A dozen of shirts to
      your back." "A dozen sons." "Half a dozen friends."
      --Shak.
      [1913 Webster]

   2. An indefinite small number. --Milton.
      [1913 Webster]

   A baker's dozen, thirteen; -- called also a long dozen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dozen
    adj 1: denoting a quantity consisting of 12 items or units [syn:
           twelve, 12, xii, dozen]
    n 1: the cardinal number that is the sum of eleven and one [syn:
         twelve, 12, XII, dozen]