The Collaborative International Dictionary of English v.0.48:

lead-in \lead-in\ n.
   1. (Elec.) a wire connecting an antenna to a receiver or a
      transmitter to a transmission line.
      [WordNet 1.5]

   2. (Radio and Television Broadcasting) The short segment of a
      program or program script introducing and immediately
      preceding another part of the program or a commercial.
      [PJC]

WordNet (r) 3.0 (2006):

lead-in
    n 1: the introductory section of a story; "it was an amusing
         lead-in to a very serious matter" [syn: lead, lead-in,
         lede]
    2: wire connecting an antenna to a receiver or a transmitter to
       a transmission line