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]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "lactation":
   chyle, chylifaction, chylification, colostrum, discharge,
   excretion, external secretion, flow, fluency, fluidity, fluidness,
   flux, fluxility, fluxion, gleet, humor, ichor, internal secretion,
   juiciness, lachryma, lacrimation, lactescence, leukorrhea,
   liquefaction, liquidness, lymph, matter, milk, milkiness, moisture,
   mucor, mucus, peccant humor, phlegm, purulence, pus, rheum,
   rheuminess, saliva, sanies, sappiness, secernment, secreta,
   secretion, serosity, serous fluid, serum, snot, succulence,
   suppuration, sweat, tear, teardrop, the whites, urine, wateriness,
   weeping