The Collaborative International Dictionary of English v.0.48:

Linkage \Link"age\ (l[i^][ng]k"[asl]j; 48), n.
   1. The act of linking; the state of being linked; also, a
      system of links.
      [1913 Webster]

   2. (Chem.) Manner of linking or of being linked; -- said of
      the union of atoms or radicals in the molecule.
      [1913 Webster]

   3. (Geom.) A system of straight lines or bars, fastened
      together by joints, and having certain of their points
      fixed in a plane. It is used to describe straight lines
      and curves in the plane.
      [1913 Webster]

   4. Any object, factor, or relationship that creates a bond or
      association between two other objects.
      [PJC]

   5. Specifically: (Genetics) The proximity of two or more
      genes on the genome of an organism that causes those genes
      to be inherited, transferred, or moved together with a
      frequency greater than for genes not associated. The
      linkage is a continuous variable, and is inversely related
      to the distance between genes on the genome.
      [PJC]

WordNet (r) 3.0 (2006):

linkage
    n 1: an associative relation
    2: (genetics) traits that tend to be inherited together as a
       consequence of an association between their genes; all of the
       genes of a given chromosome are linked (where one goes they
       all go) [syn: linkage, gene linkage]
    3: a mechanical system of rods or springs or pivots that
       transmits power or motion
    4: the act of linking things together