A ronin was a masterless samurai during the feudal period (1185–1868) of Japan. A samurai became masterless from the ruin or fall of his master (as in the case of death in a war), or after the loss of his master's favor or privilege.
The word ronin literally means "drifting person". The term originated in the Nara and Heian periods, when it originally referred to serfs who had fled or deserted their master's land. The term then became used for a samurai who had lost their masters.
Thanks to wikipedia.