A person who belongs to a group of people designated for one purpose, often profit via crimes, such as, but not limited to, blackmail, drug dealing, prostitution and racketeering. The first and most successful gangsters were the Italian mob(Mafia). For black/latino gangster, see gangsta.
I live in New York, my town is very much Mafia run. The Italian gangsters will leave you alone so long as you do not interfere with their business. If you do, you will very likely end up dead at the bottom of a lake.