Source : Webster's Revised Unabridged Dictionary (1913)

Letter \Let"ter\, n. (Teleg.)
   A telegram longer than an ordinary message sent at rates
   lower than the standard message rate in consideration of its
   being sent and delivered subject to priority in service of
   regular messages. Such telegrams are called by the Western
   Union Company {day, or night, letters} according to the time
   of sending, and by The Postal Telegraph Company {day, or
   night, lettergrams}.