The Collaborative International Dictionary of English v.0.48:

Middle \Mid"dle\ (m[i^]d"d'l), a. [OE. middel, AS. middel; akin
   to D. middel, OHG. muttil, G. mittel. [root]271. See Mid,
   a.]
   [1913 Webster]
   1. Equally distant from the extreme either of a number of
      things or of one thing; mean; medial; as, the middle house
      in a row; a middle rank or station in life; flowers of
      middle summer; men of middle age.
      [1913 Webster]

   2. Intermediate; intervening.
      [1913 Webster]

            Will, seeking good, finds many middle ends. --Sir J.
                                                  Davies.
      [1913 Webster]

   Note: Middle is sometimes used in the formation of
         self-explaining compounds; as, middle-sized,
         middle-witted.
         [1913 Webster]

   Middle Ages, the period of time intervening between the
      decline of the Roman Empire and the revival of letters.
      Hallam regards it as beginning with the sixth and ending
      with the fifteenth century.

   Middle class, in England, people who have an intermediate
      position between the aristocracy and the artisan class. It
      includes professional men, bankers, merchants, and small
      landed proprietors
      [1913 Webster]

            The middle-class electorate of Great Britain. --M.
                                                  Arnold.
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   Middle distance. (Paint.) See Middle-ground.

   Middle English. See English, n., 2.

   Middle Kingdom, China.

   Middle oil (Chem.), that part of the distillate obtained
      from coal tar which passes over between 170[deg] and
      230[deg] Centigrade; -- distinguished from the light
      oil, and the heavy oil or dead oil.

   Middle passage, in the slave trade, that part of the
      Atlantic Ocean between Africa and the West Indies.

   Middle post. (Arch.) Same as King-post.

   Middle States, New York, New Jersey, Pennsylvania, and
      Delaware; which, at the time of the formation of the
      Union, occupied a middle position between the Eastern
      States (or New England) and the Southern States. [U.S.]

   Middle term (Logic), that term of a syllogism with which
      the two extremes are separately compared, and by means of
      which they are brought together in the conclusion.
      --Brande.

   Middle tint (Paint.), a subdued or neutral tint.
      --Fairholt.

   Middle voice. (Gram.) See under Voice.

   Middle watch, the period from midnight to four a. m.; also,
      the men on watch during that time. --Ham. Nav. Encyc.

   Middle weight, a pugilist, boxer, or wrestler classed as of
      medium weight, i. e., over 140 and not over 160 lbs., in
      distinction from those classed as light weights, heavy
      weights, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Middle Ages
    n 1: the period of history between classical antiquity and the
         Italian Renaissance [syn: Middle Ages, Dark Ages]