Ronin means a samurai who has no masters or has lost his master. No longer has a clear course and is carried by wherever the waves take him.
Miyamoto Musashi is probably the most famous Ronin known.
In modern world it can be referred to various things:
1. A freelance worker
2. A spy who's been disavowed by government
3. Husbands who've been thrown out of the house by their wives
4. A pretty good movie with Robert De Niro, Sean Bean and Jean Reno.