The Collaborative International Dictionary of English v.0.48:

Robber \Rob"ber\, n.
   One who robs; in law, one who feloniously takes goods or
   money from the person of another by violence or by putting
   him in fear.
   [1913 Webster]

         Some roving robber calling to his fellows. --Milton.
   [1913 Webster]

   Syn: Thief; depredator; despoiler; plunderer; pillager;
        rifler; brigang; freebooter; pirate. See Thief.
        [1913 Webster]

   Robber crab. (Zool.)
   (a) A purse crab.
   (b) Any hermit crab.

   Robber fly. (Zool.) Same as Hornet fly, under Hornet.


   Robber gull (Zool.), a jager gull.
      [1913 Webster]

WordNet (r) 3.0 (2006):

robber
    n 1: a thief who steals from someone by threatening violence

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "robber":
   bandit, bank robber, body snatcher, booster, brigand, buccaneer,
   burglar, bushranger, cat burglar, cat man, chicken thief, con man,
   corsair, cracksman, crook, den of thieves, embezzler, filcher,
   footpad, freebooter, ganef, ghoul, grafter, grave robber,
   highway robber, highwayman, hijacker, holdup man, housebreaker,
   jewel thief, land pirate, land shark, land-grabber, larcener,
   larcenist, lifter, mugger, peculator, petty thief, pickpocket,
   pilferer, pirate, poacher, privateer, prowler, purloiner, raffles,
   rifler, road agent, sandbagger, scrounger, second-story man,
   shoplifter, sneak thief, stealer, stickup man, swindler, thief,
   train robber, white-collar thief, yegg



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ROBBER. One who commits a robbery. One who feloniously and forcibly takes
goods or money to any value from the person of another by violence or
putting him, in fear.



The Devil's Dictionary (1881-1906):

ROBBER, n.  A candid man of affairs.
    It is related of Voltaire that one night he and some traveling
companion lodged at a wayside inn.  The surroundings were suggestive,
and after supper they agreed to tell robber stories in turn.  "Once
there was a Farmer-General of the Revenues."  Saying nothing more, he
was encouraged to continue.  "That," he said, "is the story."