This probably doesn't help much in nailing it down....
An outage is major if the general public and the press would perceive it
as an Internet outage rather than a single provider outage. A
multi-provider outage counts. So does a single provider outage that
affects a large enough geographic area that the public would perceive it
as multiple outages, i.e. the Internet is down in Boston and New York and
all over the place. Even a single provider outage at a backbone that
causes outages at several ISP's in a single city would be an "Internet"
outage, especially if the backbone were to be one of the dialup modem
suppliers to nationals like Earthlink, AOL, et al.
I guess the point I am trying to make is to start from public perceptions
since we are talking about mainly the reporting of an analysis for
publicly known outages. If a SONET ring fails over then there is no
publicly known outage. But when it starts to affect multiple points then
the public takes note and it is a significant outage that we need to learn
from. Could be multiple geographical points, multiple circuits, or
multiple providers.