Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Lactic acid} (Physiol. Chem.), a sirupy, colorless fluid,
      soluble in water, with an intensely sour taste and strong
      acid reaction. There are at least three isomeric
      modifications all having the formula {C3H6O3}. Sarcolactic
      or paralactic acid occurs chiefly in dead muscle tissue,
      while ordinary lactic acid results from fermentation. 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 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}.

Source : WordNet®

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