The Collaborative International Dictionary of English v.0.48:

Lactation \Lac*ta"tion\, n.
   The secretion and yielding of milk by the mammary gland;
   giving suck.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lactation
    n 1: the period following birth during which milk is secreted;
         "lactation normally continues until weaning"
    2: the production and secretion of milk by the mammary glands
    3: feeding an infant by giving suck at the breast [syn:
       suckling, lactation]