Avant garde has become a fashionable label for the stuff of this century that has made it into our museums and caught the eyes of the critics. The term is a put-on just like most of the artwork it describes. Avant garde just means new.
But just saying something is NEW is not nearly enough for those wanting their enema of Artspeak. NEW just doesn't sound cryptic enough. While if you say avant garde it sounds more intellectual and gives one a marker for a circuitous ramble.
As to the art it describes; the avant garde should really be called the antiquated avant garde or the avant gone because all the stuff it describes in the paradigm sense, was already done by about 1917. That was over 80 years ago. Critics however, will explain why this is really all new, Modernism, Post Modernism and hundreds of other assorted isms, if you care to read it.
... If it needs a long sermon to proclaim it art it's probably bullshit.