Definitions:

1. a class of 14th century Japanese who were trained in martial arts and were hired for espionage and assassinations noun
   • The ninja were a formidable enemy.
2. a member of the ninja who were trained in martial arts and hired for espionage or sabotage or assassinations noun
   • The target didn't notice the ninja following them.
3. any person who is highly skilled in a particular field noun
   • I'm a ninja at making cookies.