The Collaborative International Dictionary of English v.0.48:

Milker \Milk"er\, n.
   1. One who milks; also, a mechanical apparatus for milking
      cows.
      [1913 Webster]

   2. A cow or other animal that gives milk.
      [1913 Webster]

WordNet (r) 3.0 (2006):

milker
    n 1: cattle that are reared for their milk [syn: dairy cattle,
         dairy cow, milch cow, milk cow, milcher, milker]