The Collaborative International Dictionary of English v.0.48:

Roger \Rog"er\, n. [From a proper name Roger.]
   A black flag with white skull and crossbones, formerly used
   by pirates; -- called also Jolly Roger and pirate flag.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

pirate flag
    n 1: a flag usually bearing a white skull and crossbones on a
         black background; indicates a pirate ship [syn: black
         flag, pirate flag, Jolly Roger, blackjack]