1660s, "process of suckling an infant," from French lactation, from Late Latin lactationem (nominative lactatio) "a suckling," noun of action from past participle stem of lactare "suckle," from lac (genitive lactis) "milk," from PIE root *glakt- (cf. Greek gala, genitive galaktos, "milk"), which, along with *melg-, accounts for words for "milk" in most Indo-European languages (the absence of a common word for it is considered a mystery). Meaning "process of secreting milk from the breasts" first recorded 1857. Middle Irish lacht, Welsh llaeth "milk" are loan words from Latin.
lactation lac·ta·tion (lāk-tā'shən)
The secretion or formation of milk by the mammary glands.
The period during which the mammary glands secrete milk.