The Collaborative International Dictionary of English v.0.48:

6 \6\ adj.
   1. one more than five; denoting a quantity consisting of six
      items or units; -- representing the number six as an
      Arabic numeral.

   Syn: six, vi, half dozen, half a dozen
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

6
    adj 1: denoting a quantity consisting of six items or units
           [syn: six, 6, vi, half dozen, half-dozen]
    n 1: the cardinal number that is the sum of five and one [syn:
         six, 6, VI, sixer, sise, Captain Hicks, half a
         dozen, sextet, sestet, sextuplet, hexad]