What are the defining characteristics of aviator glasses?
Aviators are nothing more than a marketing gimmick developed by Bausch &amp; Lomb back in the 1930s.  The Wikipedia page on them quotes:


  They are characterised by dark, often reflective lenses having an area
  two or three times the area of the eyeball, and very thin metal frames
  with double or triple bridge (so-called ″bullet hole″) and bayonet
  earpieces or flexible cable temples that hook behind the ears. The
  original design featured G-15 tempered glass lenses, transmitting 15%
  of incoming light. The large lenses are not flat but slightly convex.
  The design attempts to cover the entire range of the human eye and
  prevent as much light as possible from entering the eye from any
  angle.


It’s possible that Bausch &amp; Lomb referred to the style of sunglasses as aviators given the popularity of early aviation and barnstorming during the late 20s and early 30s. The brand really acquired notoriety and popularity during World War II when they were used by military personnel and began to get associated with flying. General Douglas MacArthur often wore them.  Postwar, the style became popular with a number of celebrities as well.