What is the definition of avant-garde?

It is a French phrase which means the forerunner. As an army advances, the units in the front are called the vanguard. However, units which have scouted ahead of the vanguard are called the "avant-garde" from the French word "avant" which means "before".

Thus the vanguard is at the cutting edge of fashion, artistic expression or technology. But the avant-garde is ahead of the vanguard. They are doing the things which have not come into vogue but will in the future.

Since it is enormously pretentious to claim that your work is avant-garde (since that means that you can foretell the future, and that you are so much cleverer than everyone else, who only understand what is going on now), most people who claim to be avant-garde is looked upon as pretentious. This is especialy true since many who claimed to be the avant-garde had no idea where the vanguard was going, and now look like poseurs.