NOUN (2)

1. a member of the ninja who were trained in martial arts and hired for espionage or sabotage or assassinations; a person skilled in ninjutsu;

2. a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations;

WordNet (r) 3.0 (2006):

ninja
    n 1: a member of the ninja who were trained in martial arts and
         hired for espionage or sabotage or assassinations; a person
         skilled in ninjutsu
    2: a class of 14th century Japanese who were trained in martial
       arts and were hired for espionage and assassinations