The Collaborative International Dictionary of English v.0.48:

xii \xii\, adj.
   The Roman number symbolizing the value twelve; denoting a
   quantity consisting of 12 items or units. Used after a noun
   it may symbolize the ordinal number; as, Superbowl XII.

   Syn: twelve, 12, dozen.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

xii
    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]