The Collaborative International Dictionary of English v.0.48:

lactose \lac"tose`\ (l[a^]k"t[=o]s`), n.
   1. (Physiol. Chem.) The main sugar present in milk, called
      also sugar of milk or milk sugar. When isolated pure
      it is obtained crystalline; it is separable from the whey
      by evaporation and crystallization. It is a disaccharide
      with the formula C12H22O11, being chemically
      4-([beta]-D-galactosido)-D-glucose. It has a slightly
      sweet taste, is dextrorotary, and is much less soluble in
      water than either cane sugar or glucose. Formerly called
      lactin. When hydrolyzed it yields glucose and galactose.
      In cells it may be hydrolyzed by the enzyme
      [beta]-galactosidase.
      [1913 Webster +PJC]

   2. (Chem.) See Galactose.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Milk \Milk\ (m[i^]lk), n. [AS. meoluc, meoloc, meolc, milc; akin
   to OFries. meloc, D. melk, G. milch, OHG. miluh, Icel.
   mj[=o]lk, Sw. mj["o]lk, Dan. melk, Goth. miluks, G. melken to
   milk, OHG. melchan, Lith. milszti, L. mulgere, Gr.
   'ame`lgein. [root]107. Cf. Milch, Emulsion, Milt soft
   roe of fishes.]
   [1913 Webster]
   1. (Physiol.) A white fluid secreted by the mammary glands of
      female mammals for the nourishment of their young,
      consisting of minute globules of fat suspended in a
      solution of casein, albumin, milk sugar, and inorganic
      salts. "White as morne milk." --Chaucer.
      [1913 Webster]

   2. (Bot.) A kind of juice or sap, usually white in color,
      found in certain plants; latex. See Latex.
      [1913 Webster]

   3. An emulsion made by bruising seeds; as, the milk of
      almonds, produced by pounding almonds with sugar and
      water.
      [1913 Webster]

   4. (Zool.) The ripe, undischarged spat of an oyster.
      [1913 Webster]

   Condensed milk. See under Condense, v. t.

   Milk crust (Med.), vesicular eczema occurring on the face
      and scalp of nursing infants. See Eczema.

   Milk fever.
      (a) (Med.) A fever which accompanies or precedes the first
          lactation. It is usually transitory.
      (b) (Vet. Surg.) A form puerperal peritonitis in cattle;
          also, a variety of meningitis occurring in cows after
          calving.

   Milk glass, glass having a milky appearance.

   Milk knot (Med.), a hard lump forming in the breast of a
      nursing woman, due to obstruction to the flow of milk and
      congestion of the mammary glands.

   Milk leg (Med.), a swollen condition of the leg, usually in
      puerperal women, caused by an inflammation of veins, and
      characterized by a white appearance occasioned by an
      accumulation of serum and sometimes of pus in the cellular
      tissue.

   Milk meats, food made from milk, as butter and cheese.
      [Obs.] --Bailey.

   Milk mirror. Same as Escutcheon, 2.

   Milk molar (Anat.), one of the deciduous molar teeth which
      are shed and replaced by the premolars.

   Milk of lime (Chem.), a watery emulsion of calcium hydrate,
      produced by macerating quicklime in water.

   Milk parsley (Bot.), an umbelliferous plant (Peucedanum
      palustre) of Europe and Asia, having a milky juice.

   Milk pea (Bot.), a genus (Galactia) of leguminous and,
      usually, twining plants.

   Milk sickness (Med.), See milk sickness in the
      vocabulary.

   Milk snake (Zool.), a harmless American snake (Ophibolus
      triangulus, or Ophibolus eximius). It is variously
      marked with white, gray, and red. Called also milk
      adder, chicken snake, house snake, etc.

   Milk sugar. (Physiol. Chem.) See Lactose, and Sugar of
      milk (below).

   Milk thistle (Bot.), an esculent European thistle (Silybum
      marianum), having the veins of its leaves of a milky
      whiteness.

   Milk thrush. (Med.) See Thrush.

   Milk tooth (Anat.), one of the temporary first set of teeth
      in young mammals; in man there are twenty.

   Milk tree (Bot.), a tree yielding a milky juice, as the cow
      tree of South America (Brosimum Galactodendron), and the
      Euphorbia balsamifera of the Canaries, the milk of both
      of which is wholesome food.

   Milk vessel (Bot.), a special cell in the inner bark of a
      plant, or a series of cells, in which the milky juice is
      contained. See Latex.

   Rock milk. See Agaric mineral, under Agaric.

   Sugar of milk. The sugar characteristic of milk; a hard
      white crystalline slightly sweet substance obtained by
      evaporation of the whey of milk. It is used in pellets and
      powder as a vehicle for homeopathic medicines, and as an
      article of diet. See Lactose.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sugar \Sug"ar\, n. [OE. sugre, F. sucre (cf. It. zucchero, Sp.
   az['u]car), fr. Ar. sukkar, assukkar, fr. Skr. [,c]arkar[=a]
   sugar, gravel; cf. Per. shakar. Cf. Saccharine, Sucrose.]
   1. A sweet white (or brownish yellow) crystalline substance,
      of a sandy or granular consistency, obtained by
      crystallizing the evaporated juice of certain plants, as
      the sugar cane, sorghum, beet root, sugar maple, etc. It
      is used for seasoning and preserving many kinds of food
      and drink. Ordinary sugar is essentially sucrose. See the
      Note below.
      [1913 Webster]

   Note: The term sugar includes several commercial grades, as
         the white or refined, granulated, loaf or lump, and the
         raw brown or muscovado. In a more general sense, it
         includes several distinct chemical compounds, as the
         glucoses, or grape sugars (including glucose proper,
         dextrose, and levulose), and the sucroses, or true
         sugars (as cane sugar). All sugars are carbohydrates.
         See Carbohydrate. The glucoses, or grape sugars, are
         ketone alcohols of the formula C6H12O6, and they turn
         the plane of polarization to the right or the left.
         They are produced from the amyloses and sucroses, as by
         the action of heat and acids of ferments, and are
         themselves decomposed by fermentation into alcohol and
         carbon dioxide. The only sugar (called acrose) as yet
         produced artificially belongs to this class. The
         sucroses, or cane sugars, are doubled glucose
         anhydrides of the formula C12H22O11. They are usually
         not fermentable as such (cf. Sucrose), and they act
         on polarized light.
         [1913 Webster]

   2. By extension, anything resembling sugar in taste or
      appearance; as, sugar of lead (lead acetate), a poisonous
      white crystalline substance having a sweet taste.
      [1913 Webster]

   3. Compliment or flattery used to disguise or render
      acceptable something obnoxious; honeyed or soothing words.
      [Colloq.]
      [1913 Webster]

   Acorn sugar. See Quercite.

   Cane sugar, sugar made from the sugar cane; sucrose, or an
      isomeric sugar. See Sucrose.

   Diabetes sugar, or Diabetic sugar (Med. Chem.), a variety
      of sugar (grape sugar or dextrose) excreted in the urine
      in diabetes mellitus; -- the presence of such a sugar in
      the urine is used to diagnose the illness.

   Fruit sugar. See under Fruit, and Fructose.

   Grape sugar, a sirupy or white crystalline sugar (dextrose
      or glucose) found as a characteristic ingredient of ripe
      grapes, and also produced from many other sources. See
      Dextrose, and Glucose.

   Invert sugar. See under Invert.

   Malt sugar, a variety of sugar isomeric with sucrose, found
      in malt. See Maltose.

   Manna sugar, a substance found in manna, resembling, but
      distinct from, the sugars. See Mannite.

   Milk sugar, a variety of sugar characteristic of fresh
      milk, and isomeric with sucrose. See Lactose.

   Muscle sugar, a sweet white crystalline substance isomeric
      with, and formerly regarded to, the glucoses. It is found
      in the tissue of muscle, the heart, liver, etc. Called
      also heart sugar. See Inosite.

   Pine sugar. See Pinite.

   Starch sugar (Com. Chem.), a variety of dextrose made by
      the action of heat and acids on starch from corn,
      potatoes, etc.; -- called also potato sugar, corn
      sugar, and, inaccurately, invert sugar. See Dextrose,
      and Glucose.

   Sugar barek, one who refines sugar.

   Sugar beet (Bot.), a variety of beet (Beta vulgaris) with
      very large white roots, extensively grown, esp. in Europe,
      for the sugar obtained from them.

   Sugar berry (Bot.), the hackberry.

   Sugar bird (Zool.), any one of several species of small
      South American singing birds of the genera Coereba,
      Dacnis, and allied genera belonging to the family
      Coerebidae. They are allied to the honey eaters.

   Sugar bush. See Sugar orchard.

   Sugar camp, a place in or near a sugar orchard, where maple
      sugar is made.

   Sugar candian, sugar candy. [Obs.]

   Sugar candy, sugar clarified and concreted or crystallized;
      candy made from sugar.

   Sugar cane (Bot.), a tall perennial grass (Saccharum
      officinarium), with thick short-jointed stems. It has
      been cultivated for ages as the principal source of sugar.


   Sugar loaf.
      (a) A loaf or mass of refined sugar, usually in the form
          of a truncated cone.
      (b) A hat shaped like a sugar loaf.
          [1913 Webster]

                Why, do not or know you, grannam, and that sugar
                loaf?                             --J. Webster.
          [1913 Webster]

   Sugar maple (Bot.), the rock maple (Acer saccharinum).
      See Maple.

   Sugar mill, a machine for pressing out the juice of the
      sugar cane, usually consisting of three or more rollers,
      between which the cane is passed.

   Sugar mite. (Zool.)
      (a) A small mite (Tyroglyphus sacchari), often found in
          great numbers in unrefined sugar.
      (b) The lepisma.

   Sugar of lead. See Sugar, 2, above.

   Sugar of milk. See under Milk.

   Sugar orchard, a collection of maple trees selected and
      preserved for purpose of obtaining sugar from them; --
      called also, sometimes, sugar bush. [U.S.] --Bartlett.

   Sugar pine (Bot.), an immense coniferous tree (Pinus
      Lambertiana) of California and Oregon, furnishing a soft
      and easily worked timber. The resinous exudation from the
      stumps, etc., has a sweetish taste, and has been used as a
      substitute for sugar.

   Sugar squirrel (Zool.), an Australian flying phalanger
      (Belideus sciureus), having a long bushy tail and a
      large parachute. It resembles a flying squirrel. See
      Illust. under Phlanger.

   Sugar tongs, small tongs, as of silver, used at table for
      taking lumps of sugar from a sugar bowl.

   Sugar tree. (Bot.) See Sugar maple, above.
      [1913 Webster]

WordNet (r) 3.0 (2006):

milk sugar
    n 1: a sugar comprising one glucose molecule linked to a
         galactose molecule; occurs only in milk; "cow's milk
         contains about 4.7% lactose" [syn: lactose, milk sugar]