The Collaborative International Dictionary of English v.0.48:

Relay \Re*lay"\ (r?-l?"), n. [F. relais (cf. OF. relais
   relaxation, discontinuance, It. rilascio release, relief,
   rilasso relay), fr. OF. relaissier to abandon, release, fr.
   L. relaxare. See Relax.]
   1. A supply of anything arranged beforehand for affording
      relief from time to time, or at successive stages;
      provision for successive relief. Specifically:
      (a) A supply of horses placced at stations to be in
          readiness to relieve others, so that a trveler may
          proceed without delay.
      (b) A supply of hunting dogs or horses kept in readiness
          at certain places to relive the tired dogs or horses,
          and to continue the pursuit of the game if it comes
          that way.
      (c) A number of men who relieve others in carrying on some
          work.
          [1913 Webster]

   2. (Elec.) In various forms of telegraphic apparatus, a
      magnet which receives the circuit current, and is caused
      by it to bring into into action the power of a local
      battery for performing the work of making the record;
      also, a similar device by which the current in one circuit
      is made to open or close another circuit in which a
      current is passing.
      [1913 Webster]

   Relay battery (Elec.), the local battery which is brought
      into use by the action of the relay magnet, or relay.
      [1913 Webster]