Major = Tournament with >100k in prize pool if I'm not mistaken, but people call major to valve sponsored tournaments
EDIT: So you can just call IEM a big tournament and you should call valve-sponsored events a major
Valve holds 2 tournaments a year.
MAJOR = VALVE SPONSORED, 1 MILLION PRIZE POOL, IN GAME ITEMS/PICK EMS, TWICE A YEAR
Also the tournament also usually has major in the title. ;)