The Collaborative International Dictionary of English v.0.48:

Inch \Inch\, n. [OE. inche, unche, AS. ynce, L. uncia the
   twelfth part, inch, ounce. See Ounce a weight.]
   [1913 Webster]
   1. A measure of length, the twelfth part of a foot, commonly
      subdivided into halves, quarters, eights, sixteenths,
      etc., as among mechanics. It was also formerly divided
      into twelve parts, called lines, and originally into three
      parts, called barleycorns, its length supposed to have
      been determined from three grains of barley placed end to
      end lengthwise. It is also sometimes called a prime ('),
      composed of twelve seconds (''), as in the duodecimal
      system of arithmetic.
      [1913 Webster]

   Note: The symbol ' is the same symbol as the light accent, or
         the "minutes" of an arc. The "seconds" symbol should
         actually have the two strokes closer than in repeated
         "minutes", but in this dictionary '' will be
         interpreted as "seconds".
         [PJC]

               12 seconds ('') make 1 inch or prime. 12 inches
               or primes (') make 1 foot.         --B.
                                                  Greenleaf.
         [1913 Webster]

   Note: The meter, the accepted scientific standard of length,
         equals 39.37 inches; the inch is equal to 2.54
         centimeters. See Metric system, and Meter.
         [1913 Webster]

   2. A small distance or degree, whether of time or space;
      hence, a critical moment; also used metaphorically of
      minor concessins in bargaining; as, he won't give an inch;
      give him an inch and he'll take a mile.
      [1913 Webster]

            Beldame, I think we watched you at an inch. --Shak.
      [1913 Webster]

   By inches, by slow degrees, gradually.

   Inch of candle. See under Candle.

   Inches of pressure, usually, the pressure indicated by so
      many inches of a mercury column, as on a steam gauge.

   Inch of water. See under Water.

   Miner's inch, (Hydraulic Mining), a unit for the
      measurement of water. See Inch of water, under Water.
      [1913 Webster]

WordNet (r) 3.0 (2006):

by inches
    adv 1: by a short distance; "they moved it by inches" [syn: by
           inches, little by little, by small degrees]