Technically, someone who is belonging to the middle class, anywhere from the bottom of it all the way up to the top, as rich as they get, so long as they are not noble, or royalty. This term more portrays towards the 16-1800's.
Now it is used to describe people who are rich, or belonging to the upper class. It can also mean kind of snobby. It is also really annoyingly spelled because the french made it up, so its basically annoying in all aspects.