The Collaborative International Dictionary of English v.0.48:

Leading \Lead"ing\, a.
   Guiding; directing; controlling; foremost; as, a leading
   motive; a leading man; a leading example. -- Lead"ing*ly,
   adv.
   [1913 Webster]

   Leading case (Law), a reported decision which has come to
      be regarded as settling the law of the question involved.
      --Abbott.

   Leading motive [a translation of G. leitmotif] (Mus.), a
      guiding theme; in the musical drama of Wagner, a marked
      melodic phrase or short passage which always accompanies
      the reappearance of a certain person, situation, abstract
      idea, or allusion in the course of the play; a sort of
      musical label. Also called leitmotif or leitmotiv.

   Leading note (Mus.), the seventh note or tone in the
      ascending major scale; the sensible note.

   Leading question, a question so framed as to guide the
      person questioned in making his reply.

   Leading strings, strings by which children are supported
      when beginning to walk.

   To be in leading strings, to be in a state of infancy or
      dependence, or under the guidance of others.

   Leading wheel, a wheel situated before the driving wheels
      of a locomotive engine.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Leitmotif \Leit"mo*tif"\, Leitmotiv
\Leit"mo*tiv"\(l[imac]t"m[-o]*t[-e]f"), n. [G., leading motive.]
   1. (Mus.) a guiding theme; in Wagnerian opera, a marked
      melodic phrase or short musical passage which always
      accompanies the reappearance of a certain person,
      situation, abstract idea, or allusion in the course of the
      play; a sort of musical label. Also called Leading
      motive.
      [1913 Webster +PJC]

   2. a recurring theme in the activities of a person or group.
      [PJC]

WordNet (r) 3.0 (2006):

leitmotiv
    n 1: a melodic phrase that accompanies the reappearance of a
         person or situation (as in Wagner's operas) [syn:
         leitmotiv, leitmotif]