A brief and hardly formal online search provided the following definitions and characteristics:
- a term used when talking about something that is pushing the limits.
- the advance group in any field, especially in the visual, literary, or musical arts, whose works are characterized chiefly by unorthodox and experimental methods.
- of or pertaining to the experimental treatment of artistic, musical, or literary material.
- unorthodox or daring; radical.
- any creative group active in the innovation and application of new concepts and techniques in a given field (especially in the arts).
- a person or group who is innovative and actively seeks new ways to do things.
Literally speaking, "avant-garde" is a French term meaning "advance guard", and has been widely interpreted as "cutting edge" (meaning new and innovative). This definition proves a difficult one, however, because something can only be "new" or "innovative" for a relatively short period of time before it comes standard practice. Thus, what was cutting edge last year/decade/century will necessarily differ from what is cutting edge today. In that sense, Beethoven (1770-1827) was an avant-garde artist because he was innovative for his time, even though contemporary ears do not hear his music as such.
In trying to do something new, avant-garde experimentation is doomed to failure in the vast majority of cases. Thus, the benefit of this type of aesthetic experimentation is almost always the journey (the learning process), and not the destination (the product). The American composer Aaron Copland (1900-1990) summarized this notion by saying, "A creator often learns as much from his miscalculations as he does from his successes" and citing "the immemorial right of the artist to be wrong" as essential to that learning process (Music and Imagination, page 76). Copland used these words to describe the development of all creators, but it is the avant-gardist who takes that notion to extremes. The price paid for that, though, is audience alienation. In their pursuit of innovation, avant-gardists often estrange their patrons. In that way, the term "avant-garde" has earned a rather negative connotation.
Another commonly accepted and often discussed distinction between pop and classical (and in this case I am extending the term "classical" to include "avant-garde") is financial: Pop music seems to consider financial success more important than classical music does. Since avant-garde music inherently challenges its listeners, it will never be able to reap the same degree of financial rewards as pop music. Nor does it try to.
Thus, when I say "the Beatles and the avant-garde", I am referring to the experimental and innovative aspects of the Beatles' output (musical and otherwise) - much of which is centered around the band's later years, when finances were no longer a concern and the band and its members were therefore free to create without concern for financial success. And, because the journey is often more important than the destination, I refer to unreleased and unpublished creative efforts that reveal the Beatles' willingness to experience and experiment, exercising that "immemorial right of the artist to be wrong".
Copland, Aaron. Music and Imagination. Harvard University Press, Cambridge, MA, 1980.