Other forms: samurais
Use the noun samurai when you're talking about the elite Japanese warriors who practiced martial arts in pre-industrial Japan.
These days you'll only see samurai in movies about Japan, and it's easy to think of them as fictional superheroes. They really did exist, though, and played an important part in Japanese history, as members of the nobility from the tenth century all the way through the 1800s. In Japanese, the word samurai means "warrior or knight."