The Collaborative International Dictionary of English v.0.48:

Lactic \Lac"tic\, a. [L. lac, lactis, milk: cf. F. lactique. See
   Lacteal, and cf. Galactic.] (Physiol. Chem.)
   Of or pertaining to milk; procured from sour milk or whey;
   as, lactic acid; lactic fermentation, etc.
   [1913 Webster]

   Lactic acid (Physiol. Chem.), a sirupy, colorless fluid,
      soluble in water, with an intensely sour taste and strong
      acid reaction. There is one center of optical activity,
      and this results in the observation of three isomeric
      modifications all having the formula C3H6O3; one is
      dextrorotatory (L-lactic acid), the other levorotatory
      (D-lactic acid), and the third an optically inactive
      mixture of the first two (DL-lactic acid); chemically it
      is 2-hydroxypropanoic acid. Sarcolactic acid or
      paralactic acid occurs chiefly in dead muscle tissue,
      while ordinary lactic acid (DL-lactic acid) results from
      fermentation, such as the fermentation of milk by lactic
      acid bacteria. The two acids are alike in having the same
      constitution (expressed by the name ethylidene lactic
      acid), but the latter is optically inactive, while
      sarcolactic acid rotates the plane of polarization to the
      right. The third acid, ethylene lactic acid, accompanies
      sarcolactic acid in the juice of flesh, and is optically
      inactive.

   Lactic ferment, an organized ferment (Bacterium lacticum
      or Bacterium lactis), which produces lactic
      fermentation, decomposing the sugar of milk into carbonic
      and lactic acids, the latter, of which renders the milk
      sour, and precipitates the casein, thus giving rise to the
      so-called spontaneous coagulation of milk.

   Lactic fermentation. See under Fermentation.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

lactic
    adj 1: of or relating to or obtained from milk (especially sour
           milk or whey); "lactic acid"; "lactic fermentation"