what makes a messenger a messenger?

some of the comments on the images of the messengers has me thinking....

what makes a bicycle messenger a messenger?

do they have to work several years?
do they need to work through several winters?

it is an issue of how much money they make?
does it depend on how skilled they are on the bike?

are those on set wage/guarantee weighed the same as those on commission?

what are the answers?

who decides?